ONE of the problems of survey research is the bias that results when the attribute being tested influences the willingness to respond. When this bias is present, the respondent group is not representative of the target population, and results may be misleading.Mail surveys are clearly more subject to this sort of bias than personal interviews, since it is a very simple matter to lay aside a questionnaire, and rather more of a problem to turn away a skilled interviewer.In our surveys we have been particularly concerned with this type of bias, since it brings our results into question . . .