There are few studies of prevalence in resource-rich countries. Prevalence has been estimated in some communities, but can vary widely among communities, and even among schools in the same community. Some examples include Australia (prevalence in school children of 13%, with a range between schools of 0% to 28%),[5] the UK (prevalence of 2%),[6] and China (prevalence of 14%, with a range of 0% to 52%).[7]

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