Don't fall into testing traps.

• Position: Marketing News

The key to successful direct marketing is effective testing, says Michele Fitzpatrick, chief marketing officer for Harte-Hanks Direct, a direct-response advertising agency. The company is a subsidiary of Harte-Hanks Inc., San Antonio, Texas. To ensure effective testing, she proposes nine tips developed through "many years of hands-on experience" with financial services market testing:

Control your control groups. The point of a control group is to develop a baseline for measuring response to your appeal. If the control doesn't mirror your target universe, the results are likely to be skewed. So if you add names to your target group during the course of a test mailing, be sure to change your control group in the same way.

Control your rollouts. Don't send your appeal to the entire target audience simultaneously. Take a statistically valid sample and send your pitch to them. If the response is satisfactory, you can then roll out future mailings exponentially.

Don't skimp on test cells. Too small a test may not produce statistically valid results. A good rule of thumb is 5,000 to 10,000 names.

Limit your variables. It's best to test variables one at a time; either that, or test everything.

External factors can skew your results. Did a...