If you’re like many marketers, you know how to determine your sales forecasts and whether your promotions are profitable, but not which customers are. Therefore it’s important to segment your customer base to determine its distribution in terms of: best shoppers, high potential shoppers, occasional shoppers (although it’s often only sale items), past shoppers who no longer buy, and prospects who’ve never bought anything. The rule of thumb is that the best 20 percent of customers generate 80 percent of revenue. In my experience, actual results tend to be even more skewed. To help you determine where you should focus your marketing efforts, here are four steps to assess your customer file.