response rates frequently encountered in survey research threaten the external validity of findings? Because the computation of unbiased sample estimates in probability sampling requires that each element in the population have a known or calculable nonzero probability of selection (Kish, 1965), °,reat caution is usually exercised in the initial selection of a sample. Respondent refusals, however, can have an effect-of unknown magnitude-on sample estimates. One way of dealing with this problem is to try to minimize the nonresponse rate. Although there has been much experimentation with methods and technologies to minimize nonresponse, 1 even successful 1 Extensive reviews of the effectiveness of these techniques for mail surveys were published by Kanuk and Berenson (1975) and Linsky (1975). Both of these articles surveyed existing knowledge of the effectiveness of followups and reminders, preliminary notification, questionnaire length, survey sponsorship, postal options, personalizaAbstract This paper attempts to aid the process of accumulating the necessary information for making more informed judgments about the effects of nonresponse under different conditions. Two measures, which permit quantifiable nonsubjective assessment of the effects of nonresponse on sample estimates, are introduced and are used to examine the effects of respondent refusals in a random-digit-dialed general population telephone survey of over 1,200 households as the response rate is increased from 74.5 percent to 86.8 percent. By applying these measures under a wide range of conditions, the adequacy of various response rates may be assessed and more rational decisions made about the costs and benefits of devoting extraordinary resources to minimizing nonresponse.

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Pacemakers and implantable cardioverter defibrillatorswere provided by Medtronic, Inc., Minneapolis, MN.

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