A serious problem that occurs when conducting a sample survey by mail is that of delayed or no response. Late respondents delay the processing of data, while nonrespondents may bias results, unless a costly personal follow-up is conducted.

A firm that specializes in mail surveys usually experiences the following schedule of replies:

and the remainder fail to return (or return too late). The firm tries to improve this return schedule by placing a dollar bill and a message of thanks in each questionnaire. In a sample of 500 questionnaires, there were 

and the remainder were not returned or arrived too late to be processed.
Test the hypotheses that (1) the overall return schedule has been improved and (2) the rate of nonrespondents has been decreased.

25% return in week 1, 20% return in week 2, 10% return in week 3,