Mail Surveys of Professional Populations

Because virtually all published research on the effect of a monetary gratuity has been in the context of public opinion polling or market research by sampling nonprofessional populations, a randomized experiment was conducted in a mail survey of 507 professional school and public librarians. A random half (Group E) had $1.00 attached as a small token of appreciation for their cooperation. The conventional questionnaire methodology was employed for Group C. In the initial mailing, the gratuity had a powerful influence on the rate of return—80% vs. 59%. Indeed, the response rate with the gratuity for the first mailing (80%) was higher than that obtained for Group C with two mailings (74%). (No gratuity was included for either group in the second mailing.) The return rate for Group E increased to 86% following the second mailing, an unusually high return rate for a mail survey even among professional populations. Even though professional populations typically have much higher response rates than the general population, a $1.00 gratuity can have a substantial effect on increasing the return rate and, consequently, can strengthen the external validity of survey findings.