Imagine we wanted to investigate whether the reason for quitting smoking affects success rate. We recruited a sample of 120 smokers. Half wanted to quit to improve their appearance (yellow teeth and bad breath, etc.), and half wanted to quit to improve their health (life expectancy, etc.). After six months we measured how successful they had been at quitting smoking. The data were skewed so we ran a Wilcoxon rank-sum test on the data (output below). Which of the following sentences correctly reports the results from the Wilcoxon rank-sum test?

A)People who wanted to quit smoking to improve their health were significantly more successful at quitting than those who wanted to quit to improve their appearance, Ws = 3245.5, z = -2.04, p = .04.
B)People who wanted to quit smoking to improve their appearance were significantly more successful at quitting than those who wanted to quit to improve their health, T = -2.04, z = 188.57, p

Question

Imagine we wanted to investigate whether the reason for quitting smoking affects success rate. We recruited a sample of 120 smokers. Half wanted to quit to improve their appearance (yellow teeth and bad breath, etc.) , and half wanted to quit to improve their health (life expectancy, etc.) . After six months we measured how successful they had been at quitting smoking. The data were skewed so we ran a Wilcoxon rank-sum test on the data (output below) . Which of the following sentences correctly reports the results from the Wilcoxon rank-sum test?

A)People who wanted to quit smoking to improve their health were significantly more successful at quitting than those who wanted to quit to improve their appearance, Ws = 3245.5, z = -2.04, p = .04. 
B)People who wanted to quit smoking to improve their appearance were significantly more successful at quitting than those who wanted to quit to improve their health, T = -2.04, z = 188.57, p < .05. 
C)This output relates to the Mann-Whitney test and tells us nothing about the Wilcoxon rank-sum test. 
D)People who wanted to quit smoking to improve their appearance were significantly more successful at quitting than those who wanted to quit to improve their health, Ws = 3245.5, z = -2.04, p = .04.

Quizplus · Accepted Answer

The correct test for comparing two independent groups on non-normally distributed data is the Wilcoxon rank-sum test (also known as the Mann-Whitney test). The output presented is consistent with the Wilcoxon rank-sum test, which compares the medians of two groups. The p-value of .04 indicates that the difference in success rate between the two groups is statistically significant at the .05 level. Therefore, the correct sentence is D: People who wanted to quit smoking to improve their appearance were significantly more successful at quitting than those who wanted to quit to improve their health, Ws = 3245.5, z = -2.04, p = .04.

Imagine We Wanted to Investigate Whether the Reason for Quitting