Predicting Response Speed in Mail Surveys

Abstract

All mail surveys encounter resistance when they arrive at their destination because they must compete with all other elements in the respondent's environment for his or her time and cooperation. This resistance varies in degree from survey to survey but the pattern of responses it generates over time is remarkably consistent for all surveys: relatively rapid returns in the early stages followed by a gradual tapering off. This pattern can be described by a simple mathematical function, the modified exponential, which relates response rates to elapsed time. The purpose of this article is to indicate how the parameters of this function can be estimated, and how it can be used to predict response speed, forecast the time needed to achieve a desired number of responses, or estimate the number of questionnaires that ought to be mailed out initially.