A new government study finds that roughly 1 in 40 American children has autism, a huge jump from the previous estimate of 1 in every 59 children.

The survey asked parents of more than 43,000 children between the ages of 3 and 17 whether or not their children had ever been diagnosed with autism or an autism spectrum disorder, or ASD, and whether the child in question still struggled with an ASD.

Study author Michael Kogan offered several explanations for the discrepancy between the previous figure from the U.S Centers for Disease Control and Prevention, and the new figures from the 2016 National Survey of Children’s Health.

First, he noted that “because there is no biological test for ASD, it is difficult to track.” And he added that different data collection methods can produce very different results.

For example, Kogan pointed out that the CDC only collected information on 8-year-olds living in 11 residential areas.