r = sqrt ( SUM ( sqr (AveragePurchaseValueByMonth - AveragePurchaseValue)) / NumberOfMonths )
Please, help me get AveragePurchaseValueByMonth and NumberOfMonths values. count (Month) gave me result for months with product was purchased, period when has no purchasing the product, not counting.
And (if it is possible) share to me examples of XYZ-analysis in QV.