Credibility alert: the following post contains assertions and speculations by yours truly that are subject to, er, different interpretations by those who actually know what the hell they’re talking about when it comes to statistics. With hat in hand, I thank reader BKsea for calling attention to some of them. I have changed some of the wording—competently, I hope—so as not to poison the minds of less wary readers, but my original faux pas are immortalized in BKsea’s comment.
Lies, Damned Lies, and…
A few days ago my colleague, Dr. Harriet Hall, posted an article about acupuncture treatment for chronic prostatitis/chronic pelvic pain syndrome. She discussed a study that had been performed in Malaysia and reported in the American Journal of Medicine. According to the investigators,
After 10 weeks of treatment, acupuncture proved almost twice as likely as sham treatment to improve CP/CPPS symptoms. Participants receiving acupuncture were 2.4-fold more likely to experience long-term benefit than were participants receiving sham acupuncture.
The primary endpoint was to be “a 6-point decrease in NIH-CSPI total score from baseline to week 10.” At week 10, 32 of 44 subjects (73%) in the acupuncture group had experienced such a decrease, compared to 21 of 45 subjects (47%) in the sham acupuncture group. Although the authors didn’t report these statistics per se, a simple “two-proportion Z-test” (Minitab) yields the following:
Sample X N Sample p
1 32 44 0.727273
2 21 45 0.466667
Difference = p (1) – p (2)
Estimate for difference: 0.260606
95% CI for difference: (0.0642303, 0.456982)
Test for difference = 0 (vs not = 0): Z = 2.60 P-Value = 0.009
Fisher’s exact test: P-Value = 0.017
Wow! A P-value of 0.009! That’s some serious statistical significance. Even Fisher’s more conservative “exact test” is substantially less than the 0.05 that we’ve come to associate with “rejecting the null hypothesis,” which in this case is that there was no difference in the proportion of subjects who had experienced a 6-point decrease in NIH-CSPI scores at 10 weeks. Surely there is a big difference between getting “real” acupuncture and getting sham acupuncture if you’ve got chronic prostatitis/chronic pelvic pain syndrome, and this study proves it!