small study in 1998 in the United Kingdom suggesting a causal link between autism and the trivalent MMR vaccine. After data included in the report was shown to be deliberately falsified, the paper was retracted, and Wakefield was struck off the medical register in the United Kingdom. It is problematic to compare autism rates over the last three decades, as the diagnostic criteria for autism have changed with each revision of the Diagnostic and Statistical Manual (DSM), which outlines which symptoms meet the criteria for an ASD diagnosis. In 1983, the DSM did not recognize PDD-NOS or Asperger's syndrome, and the criteria for autistic disorder (AD) were more restrictive. The previous edition of the DSM, DSM-IV, included autistic disorder, childhood disintegrative disorder, PDD-NOS, and Asperger's syndrome. Due to inconsistencies in diagnosis and how much is still being learnt about autism, the most recent DSM (DSM-5) only has one diagnosis, autism spectrum disorder (ASD), which encompasses each of the previous four disorders. According to the new diagnostic criteria for ASD, one must have both struggles in social communication and interaction and restricted repetitive behaviors, interests and activities (RRBs). ASD diagnoses continue to be over four times more common among boys (1 in 34) than among girls (1 in 154), and they are reported in all racial, ethnic and socioeconomic groups. Studies have been conducted in several continents (Asia, Europe and North America) that report a prevalence rate of approximately 1 to 2 percent. A 2011 study reported a 2.6 percent prevalence of autism in South Korea. Frequency Although incidence rates measure autism prevalence directly, most epidemiological studies report other frequency measures, typically point or period prevalence, or sometimes cumulative incidence. Attention is focused mostly on whether prevalence is increasing with time. Incidence and prevalence Epidemiology defines several measures of the frequency of occurrence of a disease or condition: The incidence rate of a condition is the rate at which new cases occurred per person-year, for example, "2 new cases per 1,000 person-years". The cumulative incidence is the proportion of a population that became new cases within a specified time period, for example, "1.5 per 1,000 people became new cases during 2006". The point prevalence of a condition is the proportion of a population that had the condition at a single point in time, for example, "10 cases per 1,000 people at the start of 2006". The period prevalence is the proportion