that had the condition at any time within a stated period, for example, "15 per 1,000 people had cases during 2006". When studying how conditions are caused, incidence rates are the most appropriate measure of condition frequency as they assess probability directly. However, incidence can be difficult to measure with rarer conditions such as autism. In autism epidemiology, point or period prevalence is more useful than incidence, as the condition starts long before it is diagnosed, bearing in mind genetic elements it is inherent from conception, and the gap between initiation and diagnosis is influenced by many factors unrelated to chance. Research focuses mostly on whether point or period prevalence is increasing with time; cumulative incidence is sometimes used in studies of birth cohorts. Estimation methods The three basic approaches used to estimate prevalence differ in cost and in quality of results. The simplest and cheapest method is to count known autism cases from sources such as schools and clinics, and divide by the population. This approach is likely to underestimate prevalence because it does not count children who have not been diagnosed yet, and it is likely to generate skewed statistics because some children have better access to treatment. The second method improves on the first by having investigators examine student or patient records looking for probable cases, to catch cases that have not been identified yet. The third method, which is arguably the best, screens a large sample of an entire community to identify possible cases, and then evaluates each possible case in more detail with standard diagnostic procedures. This last method typically produces the most reliable, and the highest, prevalence estimates. Frequency estimates Estimates of the prevalence of autism vary widely depending on diagnostic criteria, age of children screened, and geographical location. Most recent reviews tend to estimate a prevalence of 1–2 per 1,000 for autism and close to 6 per 1,000 for ASD; PDD-NOS is the vast majority of ASD, Asperger syndrome is about 0.3 per 1,000 and the atypical forms childhood disintegrative disorder and Rett syndrome are much rarer. A 2006 study of nearly 57,000 British nine- and ten-year-olds reported a prevalence of 3.89 per 1,000 for autism and 11.61 per 1,000 for ASD; these higher figures could be associated with broadening diagnostic criteria. Studies based on more detailed information, such as direct observation rather than examination of medical records, identify higher prevalence; this suggests