that published figures may underestimate ASD's true prevalence. A 2009 study of the children in Cambridgeshire, England used different methods to measure prevalence, and estimated that 40% of ASD cases go undiagnosed, with the two least-biased estimates of true prevalence being 11.3 and 15.7 per 1,000. A 2009 U.S. study based on 2006 data estimated the prevalence of ASD in eight-year-old children to be 9.0 per 1,000 (approximate range 8.6–9.3). A 2009 report based on the 2007 Adult Psychiatric Morbidity Survey by the National Health Service determined that the prevalence of ASD in adults was approximately 1% of the population, with a higher prevalence in males and no significant variation between age groups; these results suggest that prevalence of ASD among adults is similar to that in children and rates of autism are not increasing. Changes with time Attention has been focused on whether the prevalence of autism is increasing with time. Earlier prevalence estimates were lower, centering at about 0.5 per 1,000 for autism during the 1960s and 1970s and about 1 per 1,000 in the 1980s, as opposed to today's 23 per 1000. The number of reported cases of autism increased dramatically in the 1990s and 2000s, prompting ongoing investigations into several potential reasons: More children may have autism; that is, the true frequency of autism may have increased. There may be more complete pickup of autism (case finding), as a result of increased awareness and funding. For example, attempts to sue vaccine companies may have increased case-reporting. The diagnosis may be applied more broadly than before, as a result of the changing definition of the disorder, particularly changes in DSM-III-R and DSM-IV. An editorial error in the description of the PDD-NOS category of Autism Spectrum Disorders in the DSM-IV, in 1994, inappropriately broadened the PDD-NOS construct. The error was corrected in the DSM-IV-TR, in 2000, reversing the PDD-NOS construct back to the more restrictive diagnostic criteria requirements from the DSM-III-R. Successively earlier diagnosis in each succeeding cohort of children, including recognition in nursery (preschool), may have affected apparent prevalence but not incidence. A review of the "rising autism" figures compared to other disabilities in schools shows a corresponding drop in findings of intellectual disability. 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