Index: trunk/ppSub/src/ppSubReadoutSubtract.c
===================================================================
--- trunk/ppSub/src/ppSubReadoutSubtract.c	(revision 23535)
+++ trunk/ppSub/src/ppSubReadoutSubtract.c	(revision 23642)
@@ -74,5 +74,5 @@
     covars->data[0] = psMemIncrRefCounter(minuend->covariance);
     covars->data[1] = psMemIncrRefCounter(subtrahend->covariance);
-    outRO->covariance = psImageCovarianceSum(covars);
+    outRO->covariance = psImageCovarianceAverage(covars);
     psFree(covars);