A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data
Put last year's sales in one column and this year's sales in the next column, one row per customer. Then use this formula in the next column to get the change:=(C2-B2)/B2
Here B2 is last year's sales and C2 is this year's sales. Format the cell as a percentage.
For example, if a customer had sales of 250 last year and 200 this year, the formula gives (200-250)/250, which is -20%. That means sales for that customer fell by 20% in one year. Sort the percentage column from smallest to largest, and the customers with the biggest drops will be at the top.