24. Everett and colleagues (1997) mailed a survey to a random sample of physicians.

Half were offered $1 if they would return the questionnaire (this was the experimental group); the other half served as a control group. The point of the study was to see if even a small incentive would increase the return rate for physician surveys. The researchers report their results as follows:

Of the 300 surveys mailed to the experimental group, 39 were undeliverable, 2 were returned uncompleted, and 164 were returned completed. Thus, the response rate for the experimental group was 63% 1164>3300 - 394 = .632. Of the 300 surveys mailed to the control group, 40 were undeliverable, and 118 were returned completed.

Thus, the response rate for the control group was 45% 1118>3300 - 404 = .452.

A chi-square test comparing the response rates for the experimental and control groups found the $1 incentive had a statistically significantly improved response rate over the control group 3211, N = 5212 = 16.0, p 6 .0014.

(a) Figure the chi-square yourself (your results should be the same, within rounding error).

(b) Figure the effect size.

(c) Explain the results to

(a) and (b)

to a person who has never had a course in statistics.