Some research requires that people respond to something they re seeing a color chart, or a series of advertisements, for instance. Even some complex ideas will be hard to understand if heard over the phone, but simplified greatly when read in a mail survey. Some populations are terribly difficult or expensive to reach by phone. The cost of reaching businesspeople by phone at their place of work, for instance, dramatically increases the gulf between the costs of mail and phone. Sometimes this factor alone dictates that either the survey be done by mail.
Some surveys require people to look up and provide data from records they keep.
Surveying clergy about church attendance or giving figures, for example, often means they have to consult records to answer accurately. This is also frequently true when interviewing regional or local directors in large organizations. In these cases, it would be very difficult to get accurate answers in a phone survey. If not, the quickness and accuracy of a professional telephone interview is usually preferable to what you get through the mail, and well worth the added expenditure.