The reported increase is largely attributable to changes in diagnostic practices, referral patterns, availability of services, age at diagnosis, and public awareness. A widely cited 2002 pilot study concluded that the observed increase in autism in California cannot be explained by changes in diagnostic criteria, but a 2006 analysis found that special education data poorly measured prevalence because so many cases were undiagnosed, and that the 1994–2003 U.S. increase was associated with declines in other diagnostic categories, indicating that diagnostic substitution had occurred.