There's little disagreement that autism spectrum disorders (ASDs) are on the increase, but estimates of the actual prevalence of these disorders is the subject of sometimes acrimonious debate. One way of getting at prevalence is to ask parents whether their children have received a diagnosis of ASD from a physician or other health-care provider. According to a report from the Centers for Disease Control and Prevention (CDC)-based on data from the National Health Interview Survey (NHIS) and the National Survey of Children's Health (NSCH)-the prevalence of a parent-reported ASD diagnosis is 5.5 for every 1,000 children from the NHIS and 5.7 for every 1,000 children from the NSCH (

 2006;55[17]:481). Consistency between the two studies in their estimates indicates a high degree of reliability about prevalence.  

What is less certain is whether counting children who have received a diagnosis is an adequate representation of the true status of ASD. Consider these findings: 