Hi Chris:

Putting aside outliers for the moment, I think the formula as displayed needs to be revisited. That formula includes some double-counting because the 13 Period figures are part of the Last Year (LY) figures and the YTD figures.

I used Excel to test the formula. I assumed a product that consistently sold 20 units per month for 24 months and created 24 monthly columns. Using the formula I consistently ended up with a result over 700. Since the product only sold 240 per year, that result isn't helpful.

It's possible that I missed a bracket or an operator, but wouldn't it be simpler to just use the 13 periods (which are the current period and the previous 12)?