Results matter more than research.
Direct marketers want a cost-effective response from every insertion. Generally the first time an ad runs it gets its best response and, unless careful attention is paid to frequency, later insertions attract fewer replies. You have to decide how, with the help of copy or size variations, and gaps between insertions, response can be maintained at an acceptable level. The various media do not enjoy the same relative importance conventional marketers assign them. The effectiveness of media is often evaluated differently.