Understanding "why" sales are down or up can be done through a systematic methodology using the following formulas: sales = revenue per session x session; traffic = direct load + natural search + marketing channel; revenue per session = conversion x average order value; conversion = (product views / sessions) x (add to carts / product views) x (orders / add to carts); average order value = average item price x units per transaction.

The first step of this methodology would be to comp two time periods, then identify the area where sales are down the most, and why, either Revenue per Session or Session. If it's Sessions, then it we'd want to identify what marketing channel caused the decrease. Once this is found, then we can dig down to the Department, Category, or even product. If Traffic isn't the issue, but Revenue per Session is down, then we can determine if it's a result of Conversion or AOV. If it's Conversion, then where in the funnel did the customer drop out: product views / sessions (aka Top of Funnel) x add to carts / product views (aka Middle of Funnel) x orders / add to carts (aka Bottom of Funnel); Once the area of the funnel is identified, then we can dig down to identify where the largest variance occurred: Department, Category, or even product. If Conversion is good, but AOV is down, then we can determine if it's Average Item Price or Units per Order; Then we can dig down to identify the locations with the largest variance.

This methodology helps to identify why sales are down or up via the Product Categories reports.