Phonological Disorder

A Phonological Disorder is speech disorder that also affects clarity of speech, however it differs from an articulation disorder. A phonological disorder is characterized by error patterns which simplify the adult production. Basically, the child does not adhere to the “rules” of sound patterns for producing spoken words. Children with this disorder may produce classes of speech sounds in a different manner or place in the oral cavity. For example all back sounds may be produced at the front of the mouth (making “d” and “t” for the g and k sounds) or may leave off all final consonants such as “bi” for “big”. Phonological processes are considered a normal part of speech-language development and typically resolve on their own. If your child is difficult to understand or continues to use a pattern of errors past the age of five, your child would benefit from a speech screening by a Speech-Language Pathologist. The table below is a helpful tool in determining the clarity of speech that can be expected during the preschool years.
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AGE IN YEARS = % UNDERSTOOD BY STRANGERS
Child aged 1 yr:  25% intelligible to strangers
Child aged 2 yrs:  50% intelligible to strangers
Child aged 3 yrs: 75% intelligible to strangers
Child aged 4 yrs:  90% intelligible to strangers

References:
http://speech-language-therapy.com/index.php option=com_content&view=article&id=38:difference&catid=11:admin&Itemid=120
http://www.asha.org/public/speech/disorders/speechsounddisorders/#signs_phon)
http://www.asha.org/public/speech/disorders/speechsounddisorders/#signs_phon
http://speech-language-therapy.com/index.php?option=com_content&view=article&id=38:difference&catid=11:admin&Itemid=120