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For example, if a merchant consistently has a 7-day EPC of $120, but their 3-month EPC is $25, then I would be suspicious. This is my feeling after working in CJ for 6 years. But, make sure there aren't short-term trends in the 7-day EPC before drawing other conclusions. Changes, such as landing-page improvements, do occur in the shortterm and the EPC increase may not yet be accounted for in the 3-month averages. So the best thing to look at when comparing 7-day and 3-month EPC are the trend graphs that CJ provides for you on the homepage of your account.
A few years ago, CJ had a "reversal rate" statistic for each merchant. It showed the fraction of leads/sales that were reversed in the whole network. I found to be one of the most stats they offered, but CJ stopped providing it. They claimed it was useless (I have no idea why they thought that).
Now it is a trial-and-error thing. Just compare 7-day to 3-month EPC histograms, and select merchants who show the most consistency between the two.
With all this in mind one can easily imagine that a hosting company that is honestly accounting for affiliate payouts will find it almost impossible to break-even. Therefore we see a lot of "cost control" in the form of reversed sales on CJ.