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When a test involves just one flight, it’s easy to see that each group should have a different keycode applied to its catalogs, so when orders arrive, you can tell which group generated the order.

But if a test spans several flights, you must apply different keycodes to catalogs going to the test and control segments in each flight — even if in any given flight the test and control segments are being treated exactly the same. If you don’t, at season’s end you won’t be able to tell which group (test or control) generated which orders in those flights where test and control got the same keycode. So you won’t be able to create all-season response rates and sales-per-book numbers for your test and control groups.