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A pervasive developmental disorder not otherwise specified is one of the four autistic disorders in the DSM-5 and also was one of the five disorders classified as a pervasive developmental disorder in the DSM-IV. According to the DSM-4, PDD-NOS is a diagnosis that is used for "severe or pervasive impairment in the development of reciprocal social interaction and/or verbal and nonverbal communication skills, or when stereotyped behavior, interests, and/or activities are present, but the criteria are not met for a specific PDD" or for several other disorders. PDD-NOS includes atypical autism, because the criteria for autistic disorder are not met, for instance because of late age of onset, atypical symptomatology, or subthreshold symptomatology, or all of these. Even though PDD-NOS is considered milder than typical autism, this is not always true. While some characteristics may be milder, others may be more severe.