The first technique won't work if the sales are rounded off to the nearest pixel, which is pretty much almost always the case. The second method won't work if

The first technique won't work if the sales are rounded off to the nearest pixel, which is pretty much almost always the case. The second method won't work if the sales do not follow a Poisson distribution, for instance a product will have better sales at launch and during advertising campaigns. The sales figures don't always float around a constant average. Also, I hadn't checked the math on the second method but the units don't add up. On the LHS, you have [sales / pixel]x[pixels] = [sales] On the RHS, you have ([sales / pixel]x[pixels])² = [sales]² IIRC, the left hand side should be squared.