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US: Global warming causes birth defects…

December 23, 2012

…if you spend nine months in an airport and get your statistics wrong.

What would you name a paper which purports to discover that heat waves stress pregnant women and thus cause birth defects but which never once measures the actual exposure of pregnant women to heat?

If you answered “A Population-Based Case–Control Study of Extreme Summer Temperature and Birth Defects” then good for you, because just such a work was put into the peer-reviewed journal Environmental Health Perspectives (2012 October; 120(10): 1443–1449) by Zutphen and others…

The temperatures were measured at airports, and women were “assigned” the temperature of the nearest airport where they resided when they gave birth. Only 18 stations for the entire state of New York were used. This of course means we have little or no idea what temperatures any of the mothers actually experienced during their pregnancies (“we were unable to incorporate air conditioner use data…”). So we have no idea if excessive temperatures would have caused any birth defects for these women…

I gave up early on this one and leave it as an exercise for long-time readers to discover the many other flaws of this paper. I’ll just mention the second biggest—the first is the use of the ecological fallacy, as detailed above—which I’ll put in the authors’ own words:

Under the null hypothesis, we would expect 4 of the 84 effect estimates displayed in Table 3 to be statistically significant at the p = 0.05 level. Thus, significant positive and negative associations with cataracts, renal agenesis, and anophthalmia may have been chance findings. Bonferroni adjustment to the p = 0.05 level of significance (0.05/84 = 0.0006) would yield approximate adjusted CIs for congenital cataracts that include the null value (95% CI: 0.93, 2.44).

In other words, after using the proper (frequentist) measure, the results are not statistically significant, meaning the conclusion stated above is false. As in, Nothing to see here. No effects were found. The results were insignificant.

William M. Briggs

The alarmists are getting really, really desperate.

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