Decent estimate for variance in mpa.

pub/lib/slice.cpp

@@ -275,18 +275,24 @@ PSpec CSpec::binned( const vector<int>& breaks ) const
     for ( int iv=0; iv<breaks.size()-1; ++iv ) {
         int igi = breaks[iv];
         int igf = breaks[iv+1];
-        double xg = 0, yg = 0, vg = 0;
+        double xg = 0, yg = 0;
+        int ng = igf - igi;
         for ( int i=igi; i<igf; ++i ) {
             xg += x[i];
             yg += y[i];
+        }
+        xg /= ng;
+        yg /= ng;
+        double vg_src = 0, vg_grp = 0;
+        for ( int i=igi; i<igf; ++i ) {
             if ( has_dy() )
-                vg += SQR( dy[i] );
+                vg_src += SQR( dy[i] );
+            vg_grp += SQR( y[i]-yg );
         }
-        int ng = igf - igi;
-        if ( has_dy() )
-            sout->push_xyd(xg/ng, yg/ng, sqrt(vg)/ng );
-        else
-            sout->push_xy(xg/ng, yg/ng );
+        double vg = vg_src/ng;
+        if( ng>1 )
+            vg += vg_grp / (ng-1);
+        sout->push_xyd(xg, yg, sqrt(vg) );
     }
     return sout;
 }