Index: trunk/ippScripts/scripts/detrend_process_exp.pl
===================================================================
--- trunk/ippScripts/scripts/detrend_process_exp.pl	(revision 9446)
+++ trunk/ippScripts/scripts/detrend_process_exp.pl	(revision 9524)
@@ -61,5 +61,5 @@
 {
     my @backgrounds;		# Array of backgrounds in each component
-    my @stdevs;			# Array of standard deviations in each component
+    my @variances;    # Array of variances for each component
     foreach my $file (@$files) {
 	die "Unable to find class id\n" unless defined $file->{class_id};
@@ -68,5 +68,5 @@
 	die "Unable to find bg_mean_stdev for class_id=$class_id\n" unless defined $file->{bg_mean_stdev};
 	push @backgrounds, $file->{bg};
-	push @stdevs, $file->{bg_mean_stdev};
+	push @variances, $file->{bg_stdev}**2;
     }
 
@@ -75,14 +75,10 @@
 	$stats->add_data(@backgrounds);
 	$bg = $stats->mean();
-	if (scalar @backgrounds == 1) {
-	    $bg_stdev = 0.0;
-	} else {
-	    $bg_stdev = $stats->standard_deviation();
-	}
+	$bg_mean_stdev = $stats->standard_deviation() || 0.0;
     }
     {
 	my $stats = Statistics::Descriptive::Sparse->new; # Statistics calculator
-	$stats->add_data(@stdevs);
-	$bg_mean_stdev = $stats->mean();
+	$stats->add_data(@variances);
+	$bg_stdev = sqrt( $stats->mean() );
     }
 }