From Scientific American:
Children who live in homes with vinyl floors, which can emit chemicals called phthalates, are more likely to have autism, according to research by Swedish and U.S. scientists published Monday.
The study of Swedish children is among the first to find an apparent connection between an environmental chemical and autism.
The scientists were surprised by their finding, calling it “far from conclusive.” Because their research was not designed to focus on autism, they recommend further study of larger numbers of children to see whether the link can be confirmed. . . .
The researchers found four environmental factors associated with autism: vinyl flooring, the mother’s smoking, family economic problems and condensation on windows, which indicates poor ventilation.
Here, in a nutshell, are several of the weaknesses with the way epidemiology is currently practiced. I doubt there is anything to this, but who knows? It deserves further investigation. Here’s what could have been better:
1. The researchers did dozens of statistical tests but did not correct for the number of tests. This means there will be a high rate of false positives. The researchers appear to not quite understand this. They don’t need “further study of larger numbers” of subjects — they simply need studies of different populations. The sample size isn’t the problem; the statistical test corrects for that. It is the researchers’ failure to correct for number of tests that makes this evidence so weak.
2. They did their dozens of tests on highly correlated variables. This is like buying two of something you only need one of. A big waste. That they measured something as specific as vinyl flooring implies they gave a long questionnaire to their subjects. Perhaps there were 100 questions. Answers to those questions are likely to be highly correlated. Expensive homes tend to be different in several ways from cheaper homes. The presence/absence of vinyl flooring is likely to be correlated with family economic conditions and condensation on windows (more expensive = better ventilation). The researchers could have used factor analysis or principal components analysis to boil down their long questionnaire into a small number of factors — like 4. So instead of doing 100 tests, they could have done 4 much stronger tests. Then, if there was an unexpected correlation, there would be a good reason to take it seriously.
Someone quoted later in the article gets it completely wrong:
Dr. Philip Landrigan, a pediatrician who is director of the Children’s Environmental Health Center at Mount Sinai School of Medicine, called the results “intriguing, but in my mind preliminary because they are based on very small numbers.”
Nope. Statistical tests correct for sample size. This is like an astronomer saying the sun revolves around the earth. In this article this happens twice.
Dr. Landrigan is an epidemiologist. “Dr. Philip Landrigan has long played an important role in advancing the field of pediatric environmental health–first as an officer in the U.S. Public Health Service, then as an epidemiologist with the Centers for Disease Control and Prevention, senior advisor on children’s health to the Environmental Protection Agency and, most recently, director of the Mount Sinai Center for Children’s Health and the Environment, which was established in 1998 as the first of eight such national research centers.”
That he was called a “pediatrician” is just a bad mistake by the journalist.