--- loncom/interface/statistics/lonproblemstatistics.pm	2017/12/18 23:51:19	1.127
+++ loncom/interface/statistics/lonproblemstatistics.pm	2018/03/07 12:24:27	1.128
@@ -1,6 +1,6 @@
 # The LearningOnline Network with CAPA
 #
-# $Id: lonproblemstatistics.pm,v 1.127 2017/12/18 23:51:19 raeburn Exp $
+# $Id: lonproblemstatistics.pm,v 1.128 2018/03/07 12:24:27 raeburn Exp $
 #
 # Copyright Michigan State University Board of Trustees
 #
@@ -1672,11 +1672,11 @@ sub compute_discrimination_factor {
     my $number_to_grab = int(scalar(@{$ranking})/4);
     my $num_students = scalar(@{$ranking});
     my @BottomSet = map { $_->[&Apache::loncoursedata::RNK_student()];
-                      } @{$ranking}[0..$number_to_grab];
+                      } @{$ranking}[0..$number_to_grab-1];
     my @TopSet    =
         map {
             $_->[&Apache::loncoursedata::RNK_student()];
-          } @{$ranking}[-$number_to_grab..0];
+          } @{$ranking}[-$number_to_grab..-1];
     if (! @BottomSet || (@BottomSet == 1 && $BottomSet[0] eq '') ||
         ! @TopSet    || (@TopSet    == 1 && $TopSet[0]    eq '')) {
         return 'nan';