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In each flight (single mail drop) where test and control both got the same catalog, they applied the same mail code to the control and test segments. But in flights where the control group got a catalog and the test didn’t, the control received a unique mail code.

Staffers soon realized they couldn’t calculate separate response rates, because in flights where test and control had gotten catalogs with the same mail code, it was impossible to tell which segment had generated the order.