Marketers often wonder how many direct mail pieces they must send to get a statistically valid test. Surprisingly, it’s not so much how many pieces you mail, but how many recipients respond — a fact verified through years of testing and statistical analysis.

Depending on your degree of confidence from the test — in other words, the likelihood that the results will be repeated in subsequent mailings — you need to generate anywhere from 15 to 25 responses per test cell. If you assume a 1 percent response rate, you need to mail 2,500 pieces to get 25 replies. For a split test of two different mailings, you need to test 5,000 pieces — 2,500 per test cell.

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