Chance News 30

Supplement may help treat gambling addiction

Miniapolis Star Tribune, September 12, 2007

There seems to be a never-ending supply of questionable statistical studies. Consider the recent Minneapolis Star Tribune account of September 12, 2007. A University of Minnesota researcher publishing in the September 15, 2007 issue of Biological Psychiatry treated "27 pathological gamblers for eight weeks" with an amino acid supplement, N-acetyl cysteine . "By the end, 60 percent said they had fewer urges to gamble." Of the 16 who reported a benefit, "13 remained in a follow-up study.five out the six on the supplement reported continued improvement, compared to two out of seven on a placebo." According to the researcher, "There does seem to be some effect, but you would need bigger numbers."

Here are the results of the follow-up study as seen by Minitab:

MTB > PTwo 6 5 7 2.

Test and CI for Two Proportions

Sample X N Sample p

1 5 6 0.833333

2 2 7 0.285714

Difference = p (1) - p (2)

Estimate for difference: 0.547619

95% CI for difference: (0.0993797, 0.995858)

Test for difference = 0 (vs not = 0): Z = 2.39 P-Value = 0.017

Fisher's exact test: P-Value = 0.103

 a.. NOTE * The normal approximation may be inaccurate for small samples.

Discussion

 1.. Assume you are a frequentist, what about statistical significance?  Note the discrepancy between the exact P-Value and the P-Value using the normal approximation. 
 2.. Assume you are a Bayesian and thus immune to P-Value whether exact or due to a normal approximation, pick your priors and find the probability that there is a difference between the effect of the supplement and the effect of the placebo. 
 3.. Aside from the choice of inference procedure, frequentist or Bayesian, what other flaws do you see in this study with regard to sample size and measurement of success? 
 4.. Speculate as to why this study was reported in a Twin Cities newspaper and probably not elsewhere. 
 5.. Speculate on what might happen if the 11 who did not respond to the supplement originally were put on the follow-up study.

Submitted by Paul Alper