--- loncom/interface/statistics/lonproblemstatistics.pm	2004/06/16 16:22:52	1.89
+++ loncom/interface/statistics/lonproblemstatistics.pm	2004/07/22 15:56:41	1.91
@@ -1,6 +1,6 @@
 # The LearningOnline Network with CAPA
 #
-# $Id: lonproblemstatistics.pm,v 1.89 2004/06/16 16:22:52 matthew Exp $
+# $Id: lonproblemstatistics.pm,v 1.91 2004/07/22 15:56:41 matthew Exp $
 #
 # Copyright Michigan State University Board of Trustees
 #
@@ -704,7 +704,9 @@ sub BuildProblemStatisticsPage {
 sub output_sequence_statistics {
     my ($r) = @_;
     my $c=$r->connection();
-    $r->print('<h2>'.&mt('Sequence Statistics').'</h2>');
+    $r->print('<h2>'.&mt('Sequence Statistics').
+	      &Apache::loncommon::help_open_topic('Statistics_Sequence').
+	      '</h2>');
     $r->print('<table border="0"><tr><td bgcolor="#777777">'."\n".
               '<table border="0" cellpadding="3">'."\n".
               '<tr bgcolor="#FFFFE6">');
@@ -1579,6 +1581,10 @@ sub compute_discrimination_factor {
         map { 
             $_->[&Apache::loncoursedata::RNK_student()]; 
           } @{$ranking}[($num_students-$number_to_grab)..($num_students-1)];
+    if (! @BottomSet || (@BottomSet == 1 && $BottomSet[0] eq '') ||
+        ! @TopSet    || (@TopSet    == 1 && $TopSet[0]    eq '')) {
+        return 'nan';
+    }
     my ($bottom_sum,$bottom_max) = 
         &Apache::loncoursedata::get_sum_of_scores($resource,$part,\@BottomSet,
                                                   undef,$starttime,$endtime);