--- loncom/interface/statistics/lonproblemstatistics.pm	2005/04/07 06:56:24	1.104
+++ loncom/interface/statistics/lonproblemstatistics.pm	2006/02/05 19:10:28	1.106
@@ -1,6 +1,6 @@
 # The LearningOnline Network with CAPA
 #
-# $Id: lonproblemstatistics.pm,v 1.104 2005/04/07 06:56:24 albertel Exp $
+# $Id: lonproblemstatistics.pm,v 1.106 2006/02/05 19:10:28 bowersj2 Exp $
 #
 # Copyright Michigan State University Board of Trustees
 #
@@ -661,6 +661,11 @@ Main interface to problem statistics.
 my $navmap;
 my @sequences;
 
+sub clean_up {
+    undef($navmap);
+    undef(@sequences);
+}
+
 sub BuildProblemStatisticsPage {
     my ($r,$c)=@_;
     undef($navmap);
@@ -705,6 +710,7 @@ sub BuildProblemStatisticsPage {
                       'for the first analysis.  Future analysis this session '.
                       ' will not have this delay.').
                   '</p>');
+	&clean_up();
         return;
     }
     $r->rflush();
@@ -718,6 +724,7 @@ sub BuildProblemStatisticsPage {
     if (! ref($navmap)) {
         $r->print('<h1>'.&mt('A course-wide error occured.').'</h1>'.
                   '<h3>'.$navmap.'</h3>');
+	&clean_up();
         return;
     }
     if (exists($env{'form.Excel'})) {
@@ -765,6 +772,7 @@ sub BuildProblemStatisticsPage {
             &output_sequence_statistics($r);
         }
     }
+    &clean_up();
     return;
 }
 
@@ -1622,7 +1630,7 @@ sub compute_discrimination_factor {
         (\@Resources,
          [&Apache::lonstatistics::get_selected_sections()],
          $Apache::lonstatistics::enrollment_status,undef,
-         $starttime,$endtime);
+         $starttime,$endtime, $symb);
     #
     # compute their percent scores on the problems in the sequence,
     my $number_to_grab = int(scalar(@{$ranking})/4);
@@ -1632,7 +1640,7 @@ sub compute_discrimination_factor {
     my @TopSet    = 
         map { 
             $_->[&Apache::loncoursedata::RNK_student()]; 
-          } @{$ranking}[($num_students-$number_to_grab)..($num_students-1)];
+          } @{$ranking}[-$number_to_grab..0];
     if (! @BottomSet || (@BottomSet == 1 && $BottomSet[0] eq '') ||
         ! @TopSet    || (@TopSet    == 1 && $TopSet[0]    eq '')) {
         return 'nan';