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The essential feature of phonological disorder is failure to use developmentally expected speech sounds that are appropriate for the individual’s age and dialect. This may be present in sound production or use and may also involve substituting or omitting sounds. The difficulties usually interfere with academic or occupational achievement as well as social communication. Phonological disorder is not usually diagnosed if the behaviour can be explained by another communication disorder or if there is sensory or motor deficit or neurological condition is present. The severity ranges from little or no effect on speech intelligibility to completely unintelligible speech. The course of the disorder is variable depending on the associated causes and the severity

American Psychiatric Association: (2000) Diagnostic and Statistical Manual of Mental Disorders (4th Edition.). Text Revision. Washington, DC, American Psychiatric Association.