Looking at the numbers, you'd think the number of children with autism might have doubled — but the things that changed were a US government survey and a diagnostic manual. Slightly more than two in 100 children had an autism spectrum disorder diagnosis in 2014, up from about one in 100 between 2011 and 2013, according to a report published by the US Centers for Disease Control and Prevention. The increase reflects an important change in the way that the US government surveys people about autism. By tweaking the order of questions on the survey, more parents whose children had autism reported it.

Figuring out how many children have autism isn't easy; as with other disabilities, labels have changed over time. For instance, the most recent edition of the Diagnostic and Statistical Manual of Mental Disorders — psychiatry's Bible — eliminated Asperger's syndrome as a separate diagnosis, sweeping it instead into autism spectrum disorder. Another diagnosis, "pervasive developmental disorder not otherwise specified," was also folded into the new autism description. The new DSM was published in 2013.

And, to complicate things further, sometimes respondents have trouble understanding how to answer the questions they're asked. So last year, the CDC decided to revamp its National Health Interview Survey so that it would include more specific details on what constitutes autism spectrum disorder (ASD). The new survey also changed the order of the questions so that parents would be asked about ASD before being asked if their children have an "other developmental delay."

Because of these changes, it's likely that more parents of children with autism actually reported their diagnoses. The change in the survey also explains why the report shows a simultaneous drop in the proportion of kids whose parents marked the "other developmental delay" option; that number fell to 3.57 percent in 2014, from 4.84 percent between 2011 and 2013. The total number of children diagnosed with any developmental disability was the same between surveys, the CDC says, so some parents of children with an autism diagnosis probably switched their answer from "other development delay" to autism spectrum disorder in 2014.

Surveys like this one are intended to determine whether children with developmental disabilities are getting the care they need. They also help determine what interventions help the most. Tweaks to surveys like this aren’t something to panic over — it’s just a way of making sure kids are counted correctly.