rule of 300

Direct-mail-testing rule of thumb that, to be valid, a sample mailing must be large enough to generate 300 responses based upon the expected response rate. For example, if a test mailing is to be compared to a control package that usually generates a 5% response, the test sample must have at least 6000 pieces, so that about 300 responses will be received. A smaller sample would not be expected to produce reliable results-that is, a response rate with a high probability of being consistent across several mailings of the same test package.