Key points are not available for this paper at this time.
Don A. Dillman, James A. Christenson, Edwin H. Carpenter, Ralph M. Brooks, Increasing Mail Questionnaire Response: A Four State Comparison, American Sociological Review, Vol. 39, No. 5 (Oct., 1974), pp. 744-756
Dillman et al. (Tue,) studied this question.