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Speech Sound Disorder (SSD) is a speech disorder in which some speech sounds (called phonemes) in a child's (or, sometimes, an adult's) language are either not produced, not produced correctly, or are not used correctly. SSD in children is one of the most common forms of communication disorders reported by Speech-Language Pathologists (SLPs)[1, 2]. The prevalence of SSD varies among preschool children (between 10% and 15%)[3] and school-age children (6%)[4]. Speech and language development is approximately related to all aspects of social and educational development [5]. Children with mild speech disorders may be socially isolated, and children with severe disorders may have more difficulties in terms of health, education and vocation. Many studies cited that children with speech and language disorders are at the risk of having learning and educational problems [6-9].

Shriberg et al.(1997) have introduced a speech intelligibility scale and Percentage of Consonant Correct (PCC) scale as an index of speech development and the severity of the disorder. Generally, calculating of speech intelligibility index is according to the percentage of intelligible words in connected speech that is relatively simple and subjective criteria and will not be counted as the number of sounds error [10]. But, Shriberg and Kwiatkowski (1982) have evolved PCC as a standard criterion to evaluate the performance of phonological system in children. They recommended calculating of PCC in speech sample in order to judge about phonological abilities and the degree of disorders in children [11].