Index: /trunk/psModules/src/imcombine/pmStack.c
===================================================================
--- /trunk/psModules/src/imcombine/pmStack.c	(revision 42829)
+++ /trunk/psModules/src/imcombine/pmStack.c	(revision 42830)
@@ -1527,4 +1527,7 @@
 	    pixelVariances->data.F32[A] = pixelVariances->data.F32[B];	\
 	    pixelVariances->data.F32[B] = tempVar;			\
+	    psF32 tempWgt = expWeight->data.F32[A];		\
+	    expWeight->data.F32[A] = expWeight->data.F32[B];	\
+	    expWeight->data.F32[B] = tempWgt;					\
 	    if (expTime) {						\
 		psF32 tempExp = expTime->data.F32[A];			\
@@ -1589,4 +1592,7 @@
     psVector *pixelVariances = psVectorAlloc(stackData->n, PS_TYPE_F32);
 
+    // storage vector for exposures weights (for pixels with few inputs)
+    psVector *expWeight = psVectorAlloc(stackData->n, PS_TYPE_F32);
+
     // if we are asking for the exptime maps, generate a storage vector expTime
     psVector *expTime        = expmaps && expmaps->image ? psVectorAlloc(stackData->n, PS_TYPE_F32) : NULL;
@@ -1640,4 +1646,5 @@
 		// accumulate exposure times if required
 		if (expTime)  { expTime->data.F32[nGood] = data->exp; }
+		expWeight->data.F32[nGood] = data->exp;
 		nGood ++;
 	    }
@@ -1729,12 +1736,20 @@
 	    }
 
+	    // XXX should I use the expWeight regardless of the number of inputs??
+
 	    float sum = 0.0;
 	    float varSum = 0.0;
+	    float wtSum = 0.0;
 	    for (int n = Ns; n < Ne; n++) {
-		sum += pixelData->data.F32[n];
-		varSum += pixelVariances->data.F32[n];
-	    }
-	    float mean = sum / (float) Npt;
-	    float varValue = varSum / (float) (nGoodClip*nGoodClip);
+		psF32 wt = expWeight->data.F32[n];
+		psF32 wt2 = wt*wt;
+		sum += pixelData->data.F32[n]*wt;
+		varSum += pixelVariances->data.F32[n]*wt2;
+		wtSum += wt;
+	    }
+	    // float mean = sum / (float) Npt;
+	    // float varValue = varSum / (float) (nGoodClip*nGoodClip);
+	    float mean = sum / wtSum;
+	    float varValue = varSum / PS_SQR(wtSum);
 
 	    // alternative: calculate the stdev of the pixel values