Wednesday, September 5, 2018

Inconsistent prevalence estimates highlight studies’ flaws

The lore about autism is that prevalence rates are rising — leading many people to call it, misleadingly, an ‘epidemic.’ Even among scientists, many assume that the largest prevalence estimates are the most accurate.
But epidemiologists know that the prevalence depends greatly on the methods used in the study.
In January, for example, the National Health Interview Survey in the United States reported an autism prevalence of 2.76 percent in 2016, up slightly from 2.24 percent in 2014. However, a 2014 survey of 8-year-olds by the U.S. Centers for Disease Control and Prevention (CDC) reported a lower prevalence, at 1.69 percent. And preliminary data from a 2012 survey of 8-year-olds in South Carolina suggested a higher estimate: 3.6 percent.
What may not be obvious is that these studies varied greatly in their design, which contributed to the varying estimates of prevalence, says Eric Fombonne, professor of pediatrics at Oregon Health and Science University.
We asked Fombonne about how a study’s design influences prevalence — and in many cases, inflates the estimates.