--- loncom/interface/statistics/lonproblemstatistics.pm	2003/09/29 16:20:18	1.59
+++ loncom/interface/statistics/lonproblemstatistics.pm	2004/03/29 18:22:28	1.75
@@ -1,6 +1,6 @@
 # The LearningOnline Network with CAPA
 #
-# $Id: lonproblemstatistics.pm,v 1.59 2003/09/29 16:20:18 matthew Exp $
+# $Id: lonproblemstatistics.pm,v 1.75 2004/03/29 18:22:28 matthew Exp $
 #
 # Copyright Michigan State University Board of Trustees
 #
@@ -51,11 +51,16 @@ package Apache::lonproblemstatistics;
 
 use strict;
 use Apache::lonnet();
+use Apache::loncommon();
 use Apache::lonhtmlcommon;
 use Apache::loncoursedata;
 use Apache::lonstatistics;
 use Apache::lonlocal;
 use Spreadsheet::WriteExcel;
+use Apache::lonstathelpers();
+use Time::HiRes;
+
+my @StatsArray;
 
 ##
 ## Localization notes:
@@ -109,6 +114,14 @@ my @Fields = (
              sortable  => 'yes',
              graphable => 'yes',
              long_title => 'Maximum Number of Tries' },
+           { name   => 'min_tries',
+             title  => 'Min Tries',
+             align  => 'right',
+             color  => '#DDFFFF',
+             format => '%d',
+             sortable  => 'yes',
+             graphable => 'yes',
+             long_title => 'Minumum Number of Tries' },
            { name   => 'mean_tries',
              title  => 'Mean Tries',
              align  => 'right',
@@ -133,20 +146,11 @@ my @Fields = (
              sortable  => 'yes',
              graphable => 'yes',
              long_title => 'Skew of Number of Tries' },
-           { name   => 'deg_of_diff',
-             title  => 'DoDiff',
-             align  => 'right',
-             color  => '#DDFFFF',
-             format => '%5.2f',
-             sortable  => 'yes',
-             graphable => 'yes',
-             long_title => 'Degree of Difficulty'.
-                           '[ 1 - ((#YES+#yes) / Tries) ]'},
            { name   => 'num_solved',
              title  => '#YES',
              align  => 'right',
              color  => '#FFDDDD',
-             format => '%d',
+             format => '%4.1f',#             format => '%d',
              sortable  => 'yes',
              graphable => 'yes',
              long_title => 'Number of Students able to Solve' },
@@ -154,18 +158,35 @@ my @Fields = (
              title  => '#yes',
              align  => 'right',
              color  => '#FFDDDD',
-             format => '%d',
+             format => '%4.1f',#             format => '%d',
              sortable  => 'yes',
              graphable => 'yes',
              long_title => 'Number of Students given Override' },
-           { name   => 'per_wrong',
-             title  => '%Wrng',
+           { name   => 'num_wrong',
+             title  => '#Wrng',
              align  => 'right',
-             color  => '#FFFFE6',
+             color  => '#FFDDDD',
              format => '%4.1f',
              sortable  => 'yes',
              graphable => 'yes',
              long_title => 'Percent of students whose final answer is wrong' },
+           { name   => 'deg_of_diff',
+             title  => 'DoDiff',
+             align  => 'right',
+             color  => '#FFFFE6',
+             format => '%5.2f',
+             sortable  => 'yes',
+             graphable => 'yes',
+             long_title => 'Degree of Difficulty'.
+                           '[ 1 - ((#YES+#yes) / Tries) ]'},
+           { name   => 'deg_of_disc',
+             title  => 'DoDisc',
+             align  => 'right',
+             color  => '#FFFFE6',
+             format => '%4.2f',
+             sortable  => 'yes',
+             graphable => 'yes',
+             long_title => 'Degree of Discrimination' },
 );
 
 ###############################################
@@ -182,37 +203,17 @@ select sections, maps, and output.
 
 ###############################################
 ###############################################
-my @OutputOptions = 
-    (
-     { name  => 'problem statistics grouped by sequence',
-       value => 'HTML problem statistics grouped',
-       description => 'Output statistics for the problem parts.',
-       mode => 'html',
-       show => 'grouped',
-     },
-     { name  => 'problem statistics ungrouped',
-       value => 'HTML problem statistics ungrouped',
-       description => 'Output statistics for the problem parts.',
-       mode => 'html',
-       show => 'ungrouped',
-     },
-     { name  => 'problem statistics, Excel',
-       value => 'Excel problem statistics',
-       description => 'Output statistics for the problem parts '.
-           'in an Excel workbook',
-       mode => 'excel',
-       show => 'all',
-     },
-     );
-
 sub CreateInterface {
     my $Str = '';
+    $Str .= &Apache::lonhtmlcommon::breadcrumbs
+        (undef,'Overall Problem Statistics','Statistics_Overall_Key');
     $Str .= '<table cellspacing="5">'."\n";
     $Str .= '<tr>';
     $Str .= '<td align="center"><b>'.&mt('Sections').'</b></td>';
     $Str .= '<td align="center"><b>'.&mt('Enrollment Status').'</b></td>';
     $Str .= '<td align="center"><b>'.&mt('Sequences and Folders').'</b></td>';
-    $Str .= '<td align="center"><b>'.&mt('Output').'</b></td>';
+    $Str .= '<td rowspan="2">'.
+        &Apache::lonstathelpers::limit_by_time_form().'</td>';
     $Str .= '</tr>'."\n";
     #
     $Str .= '<tr><td align="center">'."\n";
@@ -231,22 +232,22 @@ sub CreateInterface {
     };
     $Str .= &Apache::lonstatistics::MapSelect('Maps','multiple,all',5,
                                               $only_seq_with_assessments);
-    $Str .= '</td><td>'."\n";
-    my ($html,$outputmode,$show) = 
-        &Apache::lonstatistics::CreateAndParseOutputSelector(
-                                            'statsoutputmode',
-                                            'HTML problem statistics grouped',
-                                            @OutputOptions);
-    $Str .= $html;
     $Str .= '</td></tr>'."\n";
     $Str .= '</table>'."\n";
     $Str .= '<input type="submit" name="GenerateStatistics" value="'.
         &mt('Generate Statistics').'" />';
     $Str .= '&nbsp;'x5;
+    $Str .= 'Plot '.&plot_dropdown().('&nbsp;'x10);
     $Str .= '<input type="submit" name="ClearCache" value="'.
         &mt('Clear Caches').'" />';
     $Str .= '&nbsp;'x5;
-    return ($Str,$outputmode,$show);
+    $Str .= '<input type="submit" name="UpdateCache" value="'.
+        &mt('Update Student Data').'" />';
+    $Str .= '&nbsp;'x5;
+    $Str .= '<input type="submit" name="Excel" value="'.
+        &mt('Produce Excel Output').'" />';
+    $Str .= '&nbsp;'x5;
+    return $Str;
 }
 
 ###############################################
@@ -265,237 +266,529 @@ Main interface to problem statistics.
 sub BuildProblemStatisticsPage {
     my ($r,$c)=@_;
     #
-    my ($interface,$output_mode,$show) = &CreateInterface();
+    my %Saveable_Parameters = ('Status' => 'scalar',
+                               'statsoutputmode' => 'scalar',
+                               'Section' => 'array',
+                               'StudentData' => 'array',
+                               'Maps' => 'array');
+    &Apache::loncommon::store_course_settings('statistics',
+                                              \%Saveable_Parameters);
+    &Apache::loncommon::restore_course_settings('statistics',
+                                                \%Saveable_Parameters);
+    #
+    &Apache::lonstatistics::PrepareClasslist();
+    #
+    # Clear the package variables
+    undef(@StatsArray);
+    #
+    # Finally let the user know we are here
+    my $interface = &CreateInterface();
     $r->print($interface);
-    $r->print('<input type="hidden" name="statsfirstcall" value="no" />');
     $r->print('<input type="hidden" name="sortby" value="'.$ENV{'form.sortby'}.
               '" />');
-    $r->print('<input type="hidden" name="plot" value="" />');
+    #
     if (! exists($ENV{'form.statsfirstcall'})) {
+        $r->print('<input type="hidden" name="statsfirstcall" value="yes" />');
+        $r->print('<h3>'.
+                  &mt('Press "Generate Statistics" when you are ready.').
+                  '</h3><p>'.
+                  &mt('It may take some time to update the student data '.
+                      'for the first analysis.  Future analysis this session '.
+                      ' will not have this delay.').
+                  '</p>');
         return;
+    } elsif ($ENV{'form.statsfirstcall'} eq 'yes' || 
+             exists($ENV{'form.UpdateCache'}) ||
+             exists($ENV{'form.ClearCache'}) ) {
+        $r->print('<input type="hidden" name="statsfirstcall" value="no" />');
+        &Apache::lonstatistics::Gather_Student_Data($r);
+    } else {
+        $r->print('<input type="hidden" name="statsfirstcall" value="no" />');
     }
+    $r->rflush();
     #
-    &Apache::lonstatistics::Gather_Student_Data($r);
-    #
+    # This probably does not need to be done each time we are called, but
+    # it does not slow things down noticably.
+    &Apache::loncoursedata::populate_weight_table();
     #
-    if ($output_mode eq 'html') {
-        $r->print("<h2>".
-                  $ENV{'course.'.$ENV{'request.course.id'}.'.description'}.
-                  "</h2>\n");
-        $r->print("<h3>".localtime(time)."</h3>");
-        $r->rflush();
-        if ($show eq 'grouped') {
-            &output_html_grouped_by_sequence($r);
-        } elsif ($show eq 'ungrouped') {
-            &output_html_ungrouped($r);
-        }
-    } elsif ($output_mode eq 'excel') {
-        $r->print('<h2>'.&mt('Preparing Excel Spreadsheet').'</h2>');
-        &output_excel($r);
+    if (exists($ENV{'form.Excel'})) {
+        &Excel_output($r);
     } else {
-        $r->print('<h1>'.&mt('Not implemented').'</h1>');
+        my $count = 0;
+        foreach my $seq (&Apache::lonstatistics::Sequences_with_Assess()) {
+            $count += $seq->{'num_assess'};
+        }
+        if ($count > 10) {
+            $r->print('<h2>'.
+                      &mt('Compiling statistics for [_1] problems',$count).
+                      '</h2>');
+            if ($count > 30) {
+                $r->print('<h3>'.&mt('This will take some time.').'</h3>');
+            }
+            $r->rflush();
+        }
+        #
+        my $sortby = $ENV{'form.sortby'};
+        $sortby = 'container' if (! defined($sortby) || $sortby =~ /^\s*$/);
+        my $plot = $ENV{'form.plot'};
+        if ($plot eq '' || $plot eq 'none') {
+            undef($plot);
+        }
+        if ($sortby eq 'container' && ! defined($plot)) {
+            &output_html_by_sequence($r);
+        } else {
+            if (defined($plot)) {
+                &make_plot($r,$plot);
+            }
+            &output_html_stats($r);
+        }
     }
     return;
 }
 
-###############################################
-###############################################
-
-=pod 
+##########################################################
+##########################################################
+##
+## HTML output routines
+##
+##########################################################
+##########################################################
+sub output_html_by_sequence {
+    my ($r) = @_;
+    my $c = $r->connection();
+    $r->print(&html_preamble());
+    #
+    foreach my $seq (&Apache::lonstatistics::Sequences_with_Assess()) {
+        last if ($c->aborted);
+        next if ($seq->{'num_assess'} < 1);
+        $r->print("<h3>".$seq->{'title'}."</h3>".
+                  '<table border="0"><tr><td bgcolor="#777777">'."\n".
+                  '<table border="0" cellpadding="3">'."\n".
+                  '<tr bgcolor="#FFFFE6">'.
+                  &statistics_table_header('no container')."</tr>\n");
+        my @Data = &compute_statistics_on_sequence($seq);
+        foreach my $data (@Data) {
+            $r->print('<tr>'.&statistics_html_table_data($data,
+                                                         'no container').
+                      "</tr>\n");
+        }
+        $r->print('</table>'."\n".'</table>'."\n");
+        $r->rflush();
+    }
+    return;
+}
 
-=item &output_html_grouped_by_sequence()
+sub output_html_stats {
+    my ($r)=@_;
+    &compute_all_statistics($r);
+    $r->print(&html_preamble());
+    &sort_data($ENV{'form.sortby'});
+    #
+    my $count=0;
+    foreach my $data (@StatsArray) {
+        if ($count++ % 50 == 0) {
+            $r->print("</table>\n</table>\n");
+            $r->print('<table border="0"><tr><td bgcolor="#777777">'."\n".
+                      '<table border="0" cellpadding="3">'."\n".
+                      '<tr bgcolor="#FFFFE6">'.
+                      '<tr bgcolor="#FFFFE6">'.
+                      &statistics_table_header().
+                      "</tr>\n");
+        }
+        $r->print('<tr>'.&statistics_html_table_data($data)."</tr>\n");
+    }
+    $r->print("</table>\n</table>\n");
+    return;
+}
 
-Presents the statistics data as an html table organized by the order
-the assessments appear in the course.
+sub html_preamble {
+    my $Str='';
+    $Str .= "<h2>".
+        $ENV{'course.'.$ENV{'request.course.id'}.'.description'}.
+        "</h2>\n";
+    my ($starttime,$endtime) = &Apache::lonstathelpers::get_time_limits();
+    if (defined($starttime) || defined($endtime)) {
+        # Inform the user what the time limits on the data are.
+        $Str .= '<h3>'.&mt('Statistics on submissions from [_1] to [_2]',
+                           &Apache::lonlocal::locallocaltime($starttime),
+                           &Apache::lonlocal::locallocaltime($endtime)
+                           ).'</h3>';
+    }
+    $Str .= "<h3>".&mt('Compiled on [_1]',
+                       &Apache::lonlocal::locallocaltime(time))."</h3>";
+    return $Str;
+}
 
-=cut
 
 ###############################################
 ###############################################
-sub output_html_grouped_by_sequence {
-    my ($r) = @_;
-    my $problem_num = 0;
-    #$r->print(&ProblemStatisticsLegend());
-    foreach my $sequence (&Apache::lonstatistics::Sequences_with_Assess()) {
-        next if ($sequence->{'num_assess'}<1);
-        $r->print("<h3>".$sequence->{'title'}."</h3>");
-        $r->print('<table border="0"><tr><td bgcolor="#777777">'."\n");
-        $r->print('<table border="0" cellpadding="3">'."\n");
-        $r->print('<tr bgcolor="#FFFFE6">');
-        my $Str = &statistics_table_header('no container no plots');
-        $r->print('<tr bgcolor="#FFFFE6">'.$Str."</tr>\n");
-        foreach my $resource (@{$sequence->{'contents'}}) {
-            next if ($resource->{'type'} ne 'assessment');
-            foreach my $part (@{$resource->{'parts'}}) {
-                $problem_num++;
-                my $data = &get_statistics($sequence,$resource,$part,
-                                           $problem_num);
-                my $option = '';
-                $r->print('<tr>'.&statistics_html_table_data($data,
-                                                             'no container').
-                          "</tr>\n");
+##
+## Misc HTML output routines
+##
+###############################################
+###############################################
+sub statistics_html_table_data {
+    my ($data,$options) = @_;
+    my $row = '';
+    foreach my $field (@Fields) {
+        next if ($options =~ /no $field->{'name'}/);
+        $row .= '<td bgcolor="'.$field->{'color'}.'"';
+        if (exists($field->{'align'})) {
+            $row .= ' align="'.$field->{'align'}.'"';
             }
+        $row .= '>';
+        if (exists($field->{'special'}) && $field->{'special'} eq 'link') {
+            $row .= '<a href="'.$data->{$field->{'name'}.'.link'}.'">';
         }
-        $r->print("</table>\n");
-        $r->print("</td></tr></table>\n");
-        $r->rflush();
+        if (exists($field->{'format'})) {
+            $row .= sprintf($field->{'format'},$data->{$field->{'name'}});
+        } else {
+            $row .= $data->{$field->{'name'}};
+        }
+        if (exists($field->{'special'}) && $field->{'special'} eq 'link') {
+            $row.= '</a>';
+        }
+        $row .= '</td>';
     }
-    #
-    return;
+    return $row;
 }
 
-###############################################
-###############################################
-
-=pod 
-
-=item &output_html_ungrouped()
-
-Presents the statistics data in a single html table which can be sorted by
-different columns.
+sub statistics_table_header {
+    my ($options) = @_;
+    my $header_row;
+    foreach my $field (@Fields) {
+        next if ($options =~ /no $field->{'name'}/);
+        $header_row .= '<th>';
+        if (exists($field->{'sortable'}) && $field->{'sortable'} eq 'yes') {
+            $header_row .= '<a href="javascript:'.
+                'document.Statistics.sortby.value='."'".$field->{'name'}."'".
+                    ';document.Statistics.submit();">';
+        }
+        $header_row .= &mt($field->{'title'});
+        if ($options =~ /sortable/) {
+            $header_row.= '</a>';
+        }
+        if ($options !~ /no plots/        && 
+            exists($field->{'graphable'}) && 
+            $field->{'graphable'} eq 'yes') {
+            $header_row.=' (';
+            $header_row .= '<a href="javascript:'.
+                "document.Statistics.plot.value='$field->{'name'}'".
+                    ';document.Statistics.submit();">';
+            $header_row .= &mt('plot').'</a>)';
+        }
+        $header_row .= '</th>';
+    }
+    return $header_row;
+}
 
-=cut
+####################################################
+####################################################
+##
+##    Plotting Routines
+##
+####################################################
+####################################################
+sub make_plot {
+    my ($r,$plot) = @_;
+    &compute_all_statistics($r);
+    &sort_data($ENV{'form.sortby'});
+    if ($plot eq 'degrees') {
+        &degrees_plot($r);
+    } elsif ($plot eq 'tries statistics') {
+        &tries_data_plot($r);
+    } else {
+        &make_single_stat_plot($r,$plot);
+    }
+    return;
+}
 
-###############################################
-###############################################
-sub output_html_ungrouped {
-    my ($r,$option) = @_;
+sub make_single_stat_plot {
+    my ($r,$datafield) = @_;
     #
-    if (exists($ENV{'form.plot'}) && $ENV{'form.plot'} ne '') {
-        &plot_statistics($r,$ENV{'form.plot'});
+    my $title; my $yaxis;
+    foreach my $field (@Fields) {
+        next if ($field->{'name'} ne $datafield);
+        $title = $field->{'long_title'};
+        $yaxis = $field->{'title'};
+        last;
+    }
+    if ($title eq '' || $yaxis eq '') {
+        # datafield is something we do not know enough about to plot
+        $r->print('<h3>'.
+                  &mt('Unable to plot the requested statistic.').
+                  '</h3>');
+        return;
     }
     #
-    my $problem_num = 0;
-    my $show_container = 0;
-    my $show_part = 0;
-    #$r->print(&ProblemStatisticsLegend());
-    my $sortby = undef;
-    foreach my $field (@Fields) {
-        if ($ENV{'form.sortby'} eq $field->{'name'}) {
-            $sortby = $field->{'name'};
+    # Build up the data sets to plot
+    my @Labels; 
+    my @Data;
+    my $max = 1;
+    foreach my $data (@StatsArray) {
+        push(@Labels,$data->{'problem_num'});
+        push(@Data,$data->{$datafield});
+        if ($data->{$datafield}>$max) {
+            $max = $data->{$datafield};
+        }
+    }
+    foreach (1,2,3,4,5,10,15,20,25,40,50,75,100,150,200,250,300,500,600,750,
+             1000,1500,2000,2500,3000,3500,4000,5000,7500,10000,15000,20000) {
+        if ($max <= $_) {
+            $max = $_;
+            last;
+        }
+    }
+    if ($max > 20000) {
+        $max = 10000*(int($max/10000)+1);
+    }
+    #
+    $r->print("<p>".&Apache::loncommon::DrawBarGraph($title,
+                                                     'Problem Number',
+                                                     $yaxis,
+                                                     $max,
+                                                     undef, # colors
+                                                     \@Labels,
+                                                     \@Data)."</p>\n");
+    return;
+}
+
+sub degrees_plot {
+    my ($r)=@_;
+    my $count = scalar(@StatsArray);
+    my $width = 50 + 10*$count;
+    $width = 300 if ($width < 300);
+    my $height = 300;
+    my $plot = '';
+    my $ymax = 0;
+    my $ymin = 0;
+    my @Disc; my @Diff; my @Labels;    
+    foreach my $data (@StatsArray) {
+        push(@Labels,$data->{'problem_num'});
+        my $disc = $data->{'deg_of_disc'};
+        my $diff = $data->{'deg_of_diff'};
+        push(@Disc,$disc);
+        push(@Diff,$diff);
+        #
+        $ymin = $disc if ($ymin > $disc);
+        $ymin = $diff if ($ymin > $diff);
+        $ymax = $disc if ($ymax < $disc);
+        $ymax = $diff if ($ymax < $diff);
+    }
+    #
+    # Make sure we show relevant information.
+    if ($ymin < 0) {
+        if (abs($ymin) < 0.05) {
+            $ymin = 0;
+        } else {
+            $ymin = -1;
         }
     }
-    if (! defined($sortby) || $sortby eq '' || $sortby eq 'problem_num') {
-        $sortby = 'container';
-    }
-    # If there is more than one sequence, list their titles
-    my @Sequences = &Apache::lonstatistics::Sequences_with_Assess();
-    if (@Sequences < 1) {
-        $option .= ' no container';
-    }
-    #
-    # Compile the data
-    my @Statsarray;
-    foreach my $sequence (@Sequences) {
-        next if ($sequence->{'num_assess'}<1);
-        foreach my $resource (@{$sequence->{'contents'}}) {
-            next if ($resource->{'type'} ne 'assessment');
-            foreach my $part (@{$resource->{'parts'}}) {
-                $problem_num++;
-                my $data = &get_statistics($sequence,$resource,$part,
-                                           $problem_num);
-                $show_part = 1 if ($part ne '0');
-                #
-                push (@Statsarray,$data);
-            }
+    if ($ymax > 0) {
+        if (abs($ymax) < 0.05) {
+            $ymax = 0;
+        } else {
+            $ymax = 1;
         }
     }
     #
-    # Sort the data
-    my @OutputOrder;
-    if ($sortby eq 'container') {
-        @OutputOrder = @Statsarray;
+    my $xmax = $Labels[-1];
+    if ($xmax > 50) {
+        if ($xmax % 10 != 0) {
+            $xmax = 10 * (int($xmax/10)+1);
+        }
     } else {
-        # $sortby is already defined, so we can charge ahead
-        if ($sortby =~ /^(title|part)$/i) {
-            # Alpha comparison
-            @OutputOrder = sort {
-                lc($a->{$sortby}) cmp lc($b->{$sortby}) ||
-                    lc($a->{'title'}) cmp lc($b->{'title'}) ||
-                        lc($a->{'part'}) cmp lc($b->{'part'});
-            } @Statsarray;
-        } else {
-            # Numerical comparison
-            @OutputOrder = sort {
-                my $retvalue = 0;
-                if ($b->{$sortby} eq 'nan') {
-                    if ($a->{$sortby} ne 'nan') {
-                        $retvalue = -1;
-                    } else {
-                        $retvalue = 0;
-                    }
-                }
-                if ($a->{$sortby} eq 'nan') {
-                    if ($b->{$sortby} ne 'nan') {
-                        $retvalue = 1;
-                    }
-                }
-                if ($retvalue eq '0') {
-                    $retvalue = $b->{$sortby} <=> $a->{$sortby} ||
-                                lc($a->{'title'}) <=> lc($b->{'title'}) ||
-                                lc($a->{'part'})  <=> lc($b->{'part'});
-                }
-                $retvalue;
-            } @Statsarray;
+        if ($xmax % 5 != 0) {
+            $xmax = 5 * (int($xmax/5)+1);
         }
     }
-    $option .= 'no part' if (! $show_part);
-    my $num_output = 0;
     #
-    # output the headers
-    $r->print('<table border="0"><tr><td bgcolor="#777777">'."\n");
-    $r->print('<table border="0" cellpadding="3">'."\n");
-    my $Str = &statistics_table_header($option.' sortable');
-    $r->print('<tr bgcolor="#FFFFE6">'.$Str."</tr>\n");
-    #
-    foreach my $rowdata (@OutputOrder) {
-        $num_output++;
-        if ($num_output % 25 == 0) {
-            $r->print("</table>\n</td></tr></table>\n");
-            #
-            $r->print('<table border="0"><tr><td bgcolor="#777777">'."\n");
-            $r->print('<table border="0" cellpadding="3">'."\n");
-            my $Str = &statistics_table_header($option.' sortable');
-            $r->print('<tr bgcolor="#FFFFE6">'.$Str."</tr>\n");
-            $r->rflush();
+    my $discdata .= '<data>'.join(',',@Labels).'</data>'.$/.
+                    '<data>'.join(',',@Disc).'</data>'.$/;
+    #
+    my $diffdata .= '<data>'.join(',',@Labels).'</data>'.$/.
+                    '<data>'.join(',',@Diff).'</data>'.$/;
+    #
+    $plot=<<"END";
+<gnuplot 
+    texfont="10"
+    fgcolor="x000000"
+    plottype="Cartesian"
+    font="large"
+    grid="on"
+    align="center"
+    border="on"
+    transparent="on"
+    alttag="Sample Plot"
+    samples="100"
+    bgcolor="xffffff"
+    height="$height"
+    width="$width">
+    <key 
+        pos="top right"
+        title=""
+        box="off" />
+    <title>Degree of Discrmination and Degree of Difficulty</title>
+    <axis xmin="0" ymin="$ymin" xmax="$xmax" ymax="$ymax" color="x000000" />
+    <xlabel>Problem Number</xlabel>
+    <curve 
+        linestyle="linespoints" 
+        name="DoDisc" 
+        pointtype="0" 
+        color="x000000">
+        $discdata
+    </curve>
+    <curve 
+        linestyle="linespoints" 
+        name="DoDiff" 
+        pointtype="0" 
+        color="xFF0000">
+        $diffdata
+    </curve>
+</gnuplot>
+END
+    my $plotresult = 
+        '<p>'.&Apache::lonxml::xmlparse($r,'web',$plot).'</p>'.$/;
+    $r->print($plotresult);
+    return;
+}
+
+sub tries_data_plot {
+    my ($r)=@_;
+    my $count = scalar(@StatsArray);
+    my $width = 50 + 10*$count;
+    $width = 300 if ($width < 300);
+    my $height = 300;
+    my $plot = '';
+    my @STD;  my @Mean; my @Max; my @Min;
+    my @Labels;
+    my $ymax = 5;
+    foreach my $data (@StatsArray) {
+        my $max = $data->{'mean_tries'} + $data->{'std_tries'};
+        $ymax = $max if ($ymax < $max);
+        $ymax = $max if ($ymax < $max);
+        push(@Labels,$data->{'problem_num'});
+        push(@STD,$data->{'std_tries'});
+        push(@Mean,$data->{'mean_tries'});
+    }
+    #
+    # Make sure we show relevant information.
+    my $xmax = $Labels[-1];
+    if ($xmax > 50) {
+        if ($xmax % 10 != 0) {
+            $xmax = 10 * (int($xmax/10)+1);
+        }
+    } else {
+        if ($xmax % 5 != 0) {
+            $xmax = 5 * (int($xmax/5)+1);
         }
-        $r->print('<tr>'.&statistics_html_table_data($rowdata,$option).
-                  "</tr>\n");
     }
-    $r->print("</table>\n");
-    $r->print("</td></tr></table>\n");
-    $r->rflush();
+    $ymax = int($ymax)+1+2;
     #
+    my $std_data .= '<data>'.join(',',@Labels).'</data>'.$/.
+                    '<data>'.join(',',@Mean).'</data>'.$/;
+    #
+    my $std_error_data .= '<data>'.join(',',@Labels).'</data>'.$/.
+                          '<data>'.join(',',@Mean).'</data>'.$/.
+                          '<data>'.join(',',@STD).'</data>'.$/;
+    #
+    $plot=<<"END";
+<gnuplot 
+    texfont="10"
+    fgcolor="x000000"
+    plottype="Cartesian"
+    font="large"
+    grid="on"
+    align="center"
+    border="on"
+    transparent="on"
+    alttag="Sample Plot"
+    samples="100"
+    bgcolor="xffffff"
+    height="$height"
+    width="$width">
+    <title>Mean and S.D. of Tries</title>
+    <axis xmin="0" ymin="0" xmax="$xmax" ymax="$ymax" color="x000000" />
+    <xlabel>Problem Number</xlabel>
+    <curve 
+        linestyle="yerrorbars"
+        name="S.D. Tries" 
+        pointtype="1" 
+        color="x666666">
+        $std_error_data
+    </curve>
+    <curve 
+        linestyle="points"
+        name="Mean Tries" 
+        pointtype="1" 
+        color="xCC4444">
+        $std_data
+    </curve>
+</gnuplot>
+END
+    my $plotresult = 
+        '<p>'.&Apache::lonxml::xmlparse($r,'web',$plot).'</p>'.$/;
+    $r->print($plotresult);
     return;
 }
 
+sub plot_dropdown {
+    my $current = '';
+    #
+    if (defined($ENV{'form.plot'})) {
+        $current = $ENV{'form.plot'};
+    }
+    #
+    my @Additional_Plots = (
+                            { graphable=>'yes',
+                              name => 'degrees',
+                              title => 'DoDisc and DoDiff' },
+                            { graphable=>'yes',
+                              name => 'tries statistics',
+                              title => 'Mean and S.D. of Tries' });
+    #
+    my $Str= "\n".'<select name="plot" size="1">';
+    $Str .= '<option name="none"></option>'."\n";
+    $Str .= '<option name="none2">none</option>'."\n";
+    foreach my $field (@Fields,@Additional_Plots) {
+        if (! exists($field->{'graphable'}) ||
+            $field->{'graphable'} ne 'yes') {
+            next;
+        }
+        $Str .= '<option value="'.$field->{'name'}.'"';
+        if ($field->{'name'} eq $current) {
+            $Str .= ' selected ';
+        }
+        $Str.= '>'.&mt($field->{'title'}).'</option>'."\n";
+    }
+    $Str .= '</select>'."\n";
+    return $Str;
+}
+
 ###############################################
 ###############################################
-
-=pod 
-
-=item &output_excel()
-
-Presents the statistical data in an Excel 95 compatable spreadsheet file.
-
-=cut
-
+##
+## Excel output routines
+##
 ###############################################
 ###############################################
-sub output_excel {
+sub Excel_output {
     my ($r) = @_;
+    $r->print('<h2>'.&mt('Preparing Excel Spreadsheet').'</h2>');
+    ##
+    ## Compute the statistics
+    &compute_all_statistics($r);
+    my $c = $r->connection;
+    return if ($c->aborted());
+    ##
+    ## Create the excel workbook
     my $filename = '/prtspool/'.
         $ENV{'user.name'}.'_'.$ENV{'user.domain'}.'_'.
-            time.'_'.rand(1000000000).'.xls';
-    #
-    my $excel_workbook = undef;
-    my $excel_sheet = undef;
-    #
-    my $rows_output = 0;
-    my $cols_output = 0;
+        time.'_'.rand(1000000000).'.xls';
+    my ($starttime,$endtime) = &Apache::lonstathelpers::get_time_limits();
     #
     # Create sheet
-    $excel_workbook = Spreadsheet::WriteExcel->new('/home/httpd'.$filename);
+    my $excel_workbook = Spreadsheet::WriteExcel->new('/home/httpd'.$filename);
     #
     # Check for errors
     if (! defined($excel_workbook)) {
@@ -503,7 +796,7 @@ sub output_excel {
         $r->print(&mt("Problems creating new Excel file.  ".
                   "This error has been logged.  ".
                   "Please alert your LON-CAPA administrator."));
-        return ;
+        return 0;
     }
     #
     # The excel spreadsheet stores temporary data in files, then put them
@@ -517,7 +810,12 @@ sub output_excel {
     if (length($sheetname) > 31) {
         $sheetname = substr($sheetname,0,31);
     }
-    $excel_sheet = $excel_workbook->addworksheet($sheetname);
+    my $excel_sheet = $excel_workbook->addworksheet(
+        &Apache::loncommon::clean_excel_name($sheetname));
+    ##
+    ## Begin creating excel sheet
+    ##
+    my ($rows_output,$cols_output) = (0,0);
     #
     # Put the course description in the header
     $excel_sheet->write($rows_output,$cols_output++,
@@ -544,6 +842,21 @@ sub output_excel {
     $excel_sheet->write($rows_output,$cols_output++,$sectionstring);
     $cols_output += scalar(@Sections);
     #
+    # Time restrictions
+    my $time_string;
+    if (defined($starttime)) {
+        # call localtime but not lonlocal:locallocaltime because excel probably
+        # cannot handle localized text.  Probably.
+        $time_string .= 'Data collected from '.localtime($time_string);
+        if (defined($endtime)) {
+            $time_string .= ' to '.localtime($endtime);
+        }
+        $time_string .= '.';
+    } elsif (defined($endtime)) {
+        # See note above about lonlocal:locallocaltime
+        $time_string .= 'Data collected before '.localtime($endtime).'.';
+    }
+    #
     # Put the date in there too
     $excel_sheet->write($rows_output,$cols_output++,
                         'Compiled on '.localtime(time));
@@ -551,7 +864,7 @@ sub output_excel {
     $rows_output++; 
     $cols_output=0;
     #
-    # Long Headersheaders
+    # Long Headers
     foreach my $field (@Fields) {
         next if ($field->{'name'} eq 'problem_num');
         if (exists($field->{'long_title'})) {
@@ -571,34 +884,18 @@ sub output_excel {
         $excel_sheet->write($rows_output,$cols_output++,$field->{'title'});
     }
     $rows_output++;
-    #
-    # Write the data
-    my $problem_num=0;
-    foreach my $sequence (&Apache::lonstatistics::Sequences_with_Assess()) {
-        next if ($sequence->{'num_assess'}<1);
-        foreach my $resource (@{$sequence->{'contents'}}) {
-            next if ($resource->{'type'} ne 'assessment');
-            foreach my $part (@{$resource->{'parts'}}) {
-                $cols_output=0;
-                $problem_num++;
-                my $data = &get_statistics($sequence,$resource,$part,
-                                           $problem_num);
-                #
-                if (!defined($part) || $part eq '') {
-                    $part = ' ';
-                }
-                foreach my $field (@Fields) {
-                    next if ($field->{'name'} eq 'problem_num');
-                    $excel_sheet->write($rows_output,$cols_output++,
-                                        $data->{$field->{'name'}});
-                }
-                $rows_output++;
-            }
+    foreach my $data (@StatsArray) {
+        $cols_output=0;
+        foreach my $field (@Fields) {
+            next if ($field->{'name'} eq 'problem_num');
+            $excel_sheet->write($rows_output,$cols_output++,
+                                $data->{$field->{'name'}});
         }
+        $rows_output++;
     }
     #
-    # Write the excel file
     $excel_workbook->close();
+    #
     # Tell the user where to get their excel file
     $r->print('<br />'.
               '<a href="'.$filename.'">'.
@@ -607,203 +904,131 @@ sub output_excel {
     return;
 }
 
-###############################################
-###############################################
-
-=pod 
-
-=item &statistics_html_table_data()
-
-Help function used to format the rows for HTML table output.
-
-=cut
-
-###############################################
-###############################################
-sub statistics_html_table_data {
-    my ($data,$options) = @_;
-    my $row = '';
-    foreach my $field (@Fields) {
-        next if ($options =~ /no $field->{'name'}/);
-        $row .= '<td bgcolor="'.$field->{'color'}.'"';
-        if (exists($field->{'align'})) {
-            $row .= ' align="'.$field->{'align'}.'"';
-            }
-        $row .= '>';
-        if (exists($field->{'special'}) && $field->{'special'} eq 'link') {
-            $row .= '<a href="'.$data->{$field->{'name'}.'.link'}.'">';
-        }
-        if (exists($field->{'format'})) {
-            $row .= sprintf($field->{'format'},$data->{$field->{'name'}});
-        } else {
-            $row .= $data->{$field->{'name'}};
-        }
-        if (exists($field->{'special'}) && $field->{'special'} eq 'link') {
-            $row.= '</a>';
+##################################################
+##################################################
+##
+## Statistics Gathering and Manipulation Routines
+##
+##################################################
+##################################################
+sub compute_statistics_on_sequence {
+    my ($seq) = @_;
+    my @Data;
+    foreach my $res (@{$seq->{'contents'}}) {
+        next if ($res->{'type'} ne 'assessment');
+        foreach my $part (@{$res->{'parts'}}) {
+            #
+            # This is where all the work happens
+            my $data = &get_statistics($seq,$res,$part,scalar(@StatsArray)+1);
+            push (@Data,$data);
+            push (@StatsArray,$data);
         }
-        $row .= '</td>';
     }
-    return $row;
+    return @Data;
 }
 
-sub statistics_table_header {
-    my ($options) = @_;
-    my $header_row;
-    foreach my $field (@Fields) {
-        next if ($options =~ /no $field->{'name'}/);
-        $header_row .= '<th>';
-        if ($options =~ /sortable/ && 
-            exists($field->{'sortable'}) && $field->{'sortable'} eq 'yes') {
-            $header_row .= '<a href="javascript:'.
-                'document.Statistics.sortby.value='."'".$field->{'name'}."'".
-                    ';document.Statistics.submit();">';
-        }
-        $header_row .= &mt($field->{'title'});
-        if ($options =~ /sortable/) {
-            $header_row.= '</a>';
-        }
-        if ($options !~ /no plots/        && 
-            exists($field->{'graphable'}) && 
-            $field->{'graphable'} eq 'yes') {
-            $header_row.=' (';
-            $header_row .= '<a href="javascript:'.
-                "document.Statistics.plot.value='$field->{'name'}'".
-                    ';document.Statistics.submit();">';
-            $header_row .= &mt('plot').'</a>)';
-        }
-        $header_row .= '</th>';
+sub compute_all_statistics {
+    my ($r) = @_;
+    if (@StatsArray > 0) {
+        # Assume we have already computed the statistics
+        return;
+    }
+    my $c = $r->connection;
+    foreach my $seq (&Apache::lonstatistics::Sequences_with_Assess()) {
+        last if ($c->aborted);
+        next if ($seq->{'num_assess'} < 1);
+        &compute_statistics_on_sequence($seq);
     }
-    return $header_row;
 }
 
-###############################################
-###############################################
-
-=pod 
-
-=item &plot_statistics()
-
-=cut
-
-###############################################
-###############################################
-sub plot_statistics {
-    my ($r,$datafield) = @_;
-    my @Data;
-    #
+sub sort_data {
+    my ($sortkey) = @_;
+    return if (! @StatsArray);
     #
-    my $sortfield = undef;
-    my $title = undef;
+    # Sort the data
+    my $sortby = undef;
     foreach my $field (@Fields) {
-        if ($datafield eq $field->{'name'} &&
-            exists($field->{'graphable'}) && $field->{'graphable'} eq 'yes') {
-            $sortfield = $field->{'name'};
-            $title = $field->{'long_title'};
-        }
-    }
-    return if (! defined($sortfield) || $sortfield eq '');
-    #
-    my $Max = 0;
-    my $problem_num = 0;
-    foreach my $sequence (&Apache::lonstatistics::Sequences_with_Assess()) {
-        next if ($sequence->{'num_assess'}<1);
-        foreach my $resource (@{$sequence->{'contents'}}) {
-            next if ($resource->{'type'} ne 'assessment');
-            foreach my $part (@{$resource->{'parts'}}) {
-                my $problem_number++;
-                my $data = &get_statistics($sequence,$resource,$part,
-                                           $problem_num);
-                my $value = $data->{$sortfield};
-                $Max = $value if ($Max < $value);
-                push (@Data,$value);
-            }
+        if ($sortkey eq $field->{'name'}) {
+            $sortby = $field->{'name'};
         }
     }
-    #
-    # Print out plot request
-    my $yaxis = '';
-    if ($sortfield eq 'per_wrong') {
-        $yaxis = 'Percent';
-    }
-    #
-    # Determine appropriate value for $Max
-    if ($sortfield eq 'deg_of_diff') {
-        if ($Max > 0.5) {
-            $Max = 1;
-        } elsif ($Max > 0.2) {
-            $Max = 0.5;
-        } elsif ($Max > 0.1) {
-            $Max = 0.2;
-        }
-    } elsif ($sortfield eq 'per_wrong') {
-        if ($Max > 50) {
-            $Max = 100;
-        } elsif ($Max > 25) {
-            $Max = 50;
-        } elsif ($Max > 20) {
-            $Max = 25;
-        } elsif ($Max > 10) {
-            $Max = 20;
-        } elsif ($Max > 5) {
-            $Max = 10;
+    if (! defined($sortby) || $sortby eq '' || $sortby eq 'problem_num') {
+        $sortby = 'container';
+    }
+    if ($sortby ne 'container') {
+        # $sortby is already defined, so we can charge ahead
+        if ($sortby =~ /^(title|part)$/i) {
+            # Alpha comparison
+            @StatsArray = sort {
+                lc($a->{$sortby}) cmp lc($b->{$sortby}) ||
+                lc($a->{'title'}) cmp lc($b->{'title'}) ||
+                lc($a->{'part'}) cmp lc($b->{'part'});
+            } @StatsArray;
         } else {
-            $Max = 5;
+            # Numerical comparison
+            @StatsArray = sort {
+                my $retvalue = 0;
+                if ($b->{$sortby} eq 'nan') {
+                    if ($a->{$sortby} ne 'nan') {
+                        $retvalue = -1;
+                    } else {
+                        $retvalue = 0;
+                    }
+                }
+                if ($a->{$sortby} eq 'nan') {
+                    if ($b->{$sortby} ne 'nan') {
+                        $retvalue = 1;
+                    }
+                }
+                if ($retvalue eq '0') {
+                    $retvalue = $b->{$sortby} <=> $a->{$sortby}     ||
+                            lc($a->{'title'}) <=> lc($b->{'title'}) ||
+                            lc($a->{'part'})  <=> lc($b->{'part'});
+                }
+                $retvalue;
+            } @StatsArray;
         }
     }
-    
-    $r->print("<p>".&DrawGraph(\@Data,$title,'Problem Number',$yaxis,
-                               $Max)."</p>\n");
     #
-    # Print out the data
-    $ENV{'form.sortby'} = 'Contents';
-#    &output_html_ungrouped($r);
+    # Renumber the data set
+    my $count;
+    foreach my $data (@StatsArray) {
+        $data->{'problem_num'} = ++$count;
+    }
     return;
 }
 
-###############################################
-###############################################
+########################################################
+########################################################
 
-=pod 
+=pod
 
-=item &DrawGraph()
+=item &get_statistics()
 
-=cut
+Wrapper routine from the call to loncoursedata::get_problem_statistics.  
+Calls lonstathelpers::get_time_limits() to limit the data set by time
+and &compute_discrimination_factor
 
-###############################################
-###############################################
-sub DrawGraph {
-    my ($values,$title,$xaxis,$yaxis,$Max)=@_;
-    $title = '' if (! defined($title));
-    $xaxis = '' if (! defined($xaxis));
-    $yaxis = '' if (! defined($yaxis));
-    $title = &Apache::lonnet::escape($title);
-    $xaxis = &Apache::lonnet::escape($xaxis);
-    $yaxis = &Apache::lonnet::escape($yaxis);
-    #
-    my $sendValues = join(',', @$values);
-    my $sendCount = scalar(@$values);
-    $Max =1 if ($Max < 1);
-    if ( int($Max) < $Max ) {
-        $Max++;
-        $Max = int($Max);
-    }
-    my @GData = ($title,$xaxis,$yaxis,$Max,$sendCount,$sendValues);
-    return '<IMG src="/cgi-bin/graph.png?'.
-        (join('&', @GData)).'" border="1" />';
-}
+Inputs: $sequence, $resource, $part, $problem_num
+
+Returns: Hash reference with statistics data from 
+loncoursedata::get_problem_statistics.
 
+=cut
+
+########################################################
+########################################################
 sub get_statistics {
     my ($sequence,$resource,$part,$problem_num) = @_;
     #
+    my ($starttime,$endtime) = &Apache::lonstathelpers::get_time_limits();
     my $symb = $resource->{'symb'};
     my $courseid = $ENV{'request.course.id'};
     #
-    my $students = \@Apache::lonstatistics::Students;
-    if ($Apache::lonstatistics::SelectedSections[0] eq 'all') {
-        $students = undef;
-    }
     my $data = &Apache::loncoursedata::get_problem_statistics
-                        ($students,$symb,$part,$courseid);
+                        (\@Apache::lonstatistics::SelectedSections,
+                         $Apache::lonstatistics::enrollment_status,
+                         $symb,$part,$courseid,$starttime,$endtime);
     $data->{'part'}        = $part;
     $data->{'problem_num'} = $problem_num;
     $data->{'container'}   = $sequence->{'title'};
@@ -811,78 +1036,234 @@ sub get_statistics {
     $data->{'title.link'}  = $resource->{'src'}.'?symb='.
         &Apache::lonnet::escape($resource->{'symb'});
     #
+    $data->{'deg_of_disc'} = &compute_discrimination_factor($resource,$part,$sequence);
     return $data;
 }
 
+
 ###############################################
 ###############################################
 
-=pod 
+=pod
 
-=item &ProblemStatisticsLegend()
+=item &compute_discrimination_factor()
 
-HELP  This needs to be localized, or at least generated automatically.
+Inputs: $Resource, $Sequence
+
+Returns: integer between -1 and 1
 
 =cut
 
 ###############################################
 ###############################################
-sub ProblemStatisticsLegend {
-    my $Ptr = '';
-    $Ptr = '<table border="0">';
-    $Ptr .= '<tr><td>';
-    $Ptr .= '<b>#Stdnts</b></td>';
-    $Ptr .= '<td>Total number of students attempted the problem.';
-    $Ptr .= '</td></tr><tr><td>';
-    $Ptr .= '<b>Tries</b></td>';
-    $Ptr .= '<td>Total number of tries for solving the problem.';
-    $Ptr .= '</td></tr><tr><td>';
-    $Ptr .= '<b>Max Tries</b></td>';
-    $Ptr .= '<td>Largest number of tries for solving the problem by a student.';
-    $Ptr .= '</td></tr><tr><td>';
-    $Ptr .= '<b>Mean</b></td>';
-    $Ptr .= '<td>Average number of tries. [ Tries / #Stdnts ]';
-    $Ptr .= '</td></tr><tr><td>';
-    $Ptr .= '<b>#YES</b></td>';
-    $Ptr .= '<td>Number of students solved the problem correctly.';
-    $Ptr .= '</td></tr><tr><td>';
-    $Ptr .= '<b>#yes</b></td>';
-    $Ptr .= '<td>Number of students solved the problem by override.';
-    $Ptr .= '</td></tr><tr><td>';
-    $Ptr .= '<b>%Wrong</b></td>';
-    $Ptr .= '<td>Percentage of students who tried to solve the problem ';
-    $Ptr .= 'but is still incorrect. [ 100*((#Stdnts-(#YES+#yes))/#Stdnts) ]';
-    $Ptr .= '</td></tr><tr><td>';
-    $Ptr .= '<b>DoDiff</b></td>';
-    $Ptr .= '<td>Degree of Difficulty of the problem.  ';
-    $Ptr .= '[ 1 - ((#YES+#yes) / Tries) ]';
-    $Ptr .= '</td></tr><tr><td>';
-    $Ptr .= '<b>S.D.</b></td>';
-    $Ptr .= '<td>Standard Deviation of the tries.  ';
-    $Ptr .= '[ sqrt(sum((Xi - Mean)^2)) / (#Stdnts-1) ';
-    $Ptr .= 'where Xi denotes every student\'s tries ]';
-    $Ptr .= '</td></tr><tr><td>';
-    $Ptr .= '<b>Skew.</b></td>';
-    $Ptr .= '<td>Skewness of the students tries.';
-    $Ptr .= '[(sqrt( sum((Xi - Mean)^3) / #Stdnts)) / (S.D.^3)]';
-    $Ptr .= '</td></tr><tr><td>';
-    $Ptr .= '<b>Dis.F.</b></td>';
-    $Ptr .= '<td>Discrimination Factor: A Standard for evaluating the ';
-    $Ptr .= 'problem according to a Criterion<br>';
-    $Ptr .= '<b>[Criterion to group students into %27 Upper Students - ';
-    $Ptr .= 'and %27 Lower Students]</b><br>';
-    $Ptr .= '<b>1st Criterion</b> for Sorting the Students: ';
-    $Ptr .= '<b>Sum of Partial Credit Awarded / Total Number of Tries</b><br>';
-    $Ptr .= '<b>2nd Criterion</b> for Sorting the Students: ';
-    $Ptr .= '<b>Total number of Correct Answers / Total Number of Tries</b>';
-    $Ptr .= '</td></tr>';
-    $Ptr .= '<tr><td><b>Disc.</b></td>';
-    $Ptr .= '<td>Number of Students had at least one discussion.';
-    $Ptr .= '</td></tr></table>';
-    return $Ptr;
+sub compute_discrimination_factor {
+    my ($resource,$part,$sequence) = @_;
+    my @Resources;
+    foreach my $res (@{$sequence->{'contents'}}) {
+        next if ($res->{'symb'} eq $resource->{'symb'});
+        push (@Resources,$res->{'symb'});
+    }
+    #
+    # rank
+    my $ranking = 
+        &Apache::loncoursedata::rank_students_by_scores_on_resources
+        (\@Resources,
+         \@Apache::lonstatistics::SelectedSections,
+         $Apache::lonstatistics::enrollment_status,undef);
+    #
+    # compute their percent scores on the problems in the sequence,
+    my $number_to_grab = int(scalar(@{$ranking})/4);
+    my $num_students = scalar(@{$ranking});
+    my @BottomSet = map { $_->[&Apache::loncoursedata::RNK_student()]; 
+                      } @{$ranking}[0..$number_to_grab];
+    my @TopSet    = 
+        map { 
+            $_->[&Apache::loncoursedata::RNK_student()]; 
+          } @{$ranking}[($num_students-$number_to_grab)..($num_students-1)];
+    my ($bottom_sum,$bottom_max) = 
+        &Apache::loncoursedata::get_sum_of_scores($resource,$part,\@BottomSet);
+    my ($top_sum,$top_max) = 
+        &Apache::loncoursedata::get_sum_of_scores($resource,$part,\@TopSet);
+    my $deg_of_disc;
+    if ($top_max == 0 || $bottom_max==0) {
+        $deg_of_disc = 'nan';
+    } else {
+        $deg_of_disc = ($top_sum/$top_max) - ($bottom_sum/$bottom_max);
+    }
+    #&Apache::lonnet::logthis('    '.$top_sum.'/'.$top_max.
+    #                         ' - '.$bottom_sum.'/'.$bottom_max);
+    return $deg_of_disc;
+}
+
+###############################################
+###############################################
+
+=pod 
+
+=item ProblemStatisticsLegend
+
+=over 4
+
+=item #Stdnts
+Total number of students attempted the problem.
+
+=item Tries
+Total number of tries for solving the problem.
+
+=item Max Tries
+Largest number of tries for solving the problem by a student.
+
+=item Mean
+Average number of tries. [ Tries / #Stdnts ]
+
+=item #YES
+Number of students solved the problem correctly.
+
+=item #yes
+Number of students solved the problem by override.
+
+=item %Wrong
+Percentage of students who tried to solve the problem 
+but is still incorrect. [ 100*((#Stdnts-(#YES+#yes))/#Stdnts) ]
+
+=item DoDiff
+Degree of Difficulty of the problem.  
+[ 1 - ((#YES+#yes) / Tries) ]
+
+=item S.D.
+Standard Deviation of the tries.  
+[ sqrt(sum((Xi - Mean)^2)) / (#Stdnts-1) 
+where Xi denotes every student\'s tries ]
+
+=item Skew.
+Skewness of the students tries.
+[(sqrt( sum((Xi - Mean)^3) / #Stdnts)) / (S.D.^3)]
+
+=item Dis.F.
+Discrimination Factor: A Standard for evaluating the 
+problem according to a Criterion<br>
+
+=item [Criterion to group students into %27 Upper Students - 
+and %27 Lower Students]
+1st Criterion for Sorting the Students: 
+Sum of Partial Credit Awarded / Total Number of Tries
+2nd Criterion for Sorting the Students: 
+Total number of Correct Answers / Total Number of Tries
+
+=item Disc.
+Number of Students had at least one discussion.
+
+=back
+
+=cut
+
+
+############################################################
+############################################################
+##
+##  How this all works:
+##     Statistics are computed by calling &get_statistics with the sequence,
+##     resource, and part id to run statistics on.  At various places within
+##     the loops which compute the statistics, as well as before and after 
+##     the entire process, subroutines can be called.  The subroutines are
+##     registered to the following hooks:
+##
+##         hook          subroutine inputs
+##     ----------------------------------------------------------
+##         pre           $r,$count
+##         pre_seq       $r,$count,$seq
+##         pre_res       $r,$count,$seq,$res
+##         calc          $r,$count,$seq,$res,$data
+##         post_res      $r,$count,$seq,$res
+##         post_seq      $r,$count,$seq
+##         post          $r,$count
+##
+##         abort         $r
+##
+##     subroutines will be called in the order in which they are registered.
+##   
+############################################################
+############################################################
+{
+
+my %hooks;
+my $aborted = 0;
+
+sub abort_computation {
+    $aborted = 1;
+}
+
+sub clear_hooks {
+    $aborted = 0;
+    undef(%hooks);
 }
 
-#---- END Problem Statistics Web Page ----------------------------------------
+sub register_hook {
+    my ($hookname,$subref)=@_;
+    if ($hookname !~ /^(pre|pre_seq|pre_res|post|post_seq|post_res|calc)$/){
+        return;
+    }
+    if (ref($subref) ne 'CODE') {
+        &Apache::lonnet::logthis('attempt to register hook to non-code: '.
+                                 $hookname,' = '.$subref);
+    } else {
+        if (exists($hooks{$hookname})) {
+            push(@{$hooks{$hookname}},$subref);
+        } else {
+            $hooks{$hookname} = [$subref];
+        }
+    }
+    return;
+}
+
+sub run_hooks {
+    my $context = shift();
+    foreach my $hook (@{$hooks{$context}}) { 
+        if ($aborted && $context ne 'abort') {
+            last;
+        }
+        my $retvalue = $hook->(@_);
+        if (defined($retvalue) && $retvalue eq '0') {
+            $aborted = 1 if (! $aborted);
+        }
+    }
+}
+
+sub run_statistics {
+    my ($r) = @_;
+    my $count = 0;
+    &run_hooks('pre',$r,$count);
+    foreach my $seq (&Apache::lonstatistics::Sequences_with_Assess()) {
+        last if ($aborted);
+        next if ($seq->{'num_assess'}<1);
+        &run_hooks('pre_seq',$r,$count,$seq);
+        foreach my $res (@{$seq->{'contents'}}) {
+            last if ($aborted);
+            next if ($res->{'type'} ne 'assessment');
+            &run_hooks('pre_res',$r,$count,$seq,$res);            
+            foreach my $part (@{$res->{'parts'}}) {
+                last if ($aborted);
+                #
+                # This is where all the work happens
+                my $data = &get_statistics($seq,$res,$part,++$count);
+                &run_hooks('calc',$r,$count,$seq,$res,$part,$data); 
+            }
+            &run_hooks('post_res',$r,$count,$seq,$res);
+        }
+        &run_hooks('post_seq',$r,$count,$seq);
+    }
+    if ($aborted) {
+        &run_hooks('abort',$r);
+    } else {
+        &run_hooks('post',$r,$count);
+    }
+    return;
+}
+
+} # End of %hooks scope
+
+############################################################
+############################################################
 
 1;
 __END__