When designing a mail questionnaire, the researcher must always keep in mind the trade-off between data quantity and data quality. If a researcher attempts to obtain too much information in a mail survey, the percentage of people responding will drop. This has two undesirable effects:

• The responses obtained in the survey will be less representative of the "population" being measured,
• Survey costs will increase, since more questionnaires will have to be mailed to obtain an adequate number of responses.

The appropriate questionnaire length depends upon the importance (to the respondent) of the questionnaire subject. Certain transactions, such as the purchase of a home or a car, are so important to consumers that a questionnaire length of four pages is reasonable. Other transactions, such as the purchase of gasoline, are so routine in the minds of most consumers that to go beyond a half a page would be pushing it. The rule of thumb is to keep questionnaire length as short as possible. We believe that most issues addressed by mail questionnaires can be handled in one or two pages, and that there are very few circumstances justifying questionnaire lengths of more than four pages.

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