--- loncom/interface/statistics/lonproblemstatistics.pm	2004/07/12 19:30:12	1.90
+++ 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 1.90 2004/07/12 19:30:12 matthew Exp $
+# $Id: lonproblemstatistics.pm,v 1.91 2004/07/22 15:56:41 matthew Exp $
 #
 # Copyright Michigan State University Board of Trustees
 #
@@ -1581,6 +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);