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The systematic mispronunciations of young children often resemble phonological rules, and there is a temptation to treat the data as identical to adult phonological data. However, performance factors are often evident in the child's speech. Constraints in phonological theory are "hard" (all-or-nothing), but constraints in performance are often "soft" (allowing something to occur under some conditions but not under others). One child phonological process that has an obvious linguistic interpretation is compensatory lengthening, wherein a segment becomes long when a nearby segment is deleted. In data from one child, two compensatory lengthening processes led to the creation of [i:] but never [u:], even though [u:] appeared in the child's speech as the correct realization of adult /u:/. The child also showed later mastery of all back rounded vowels and glides than of the corresponding front vowels and glides. It is argued that compensatory lengthening never led to [u:] because of a soft performance constraint against back rounded segments; the hard constraints of phonological theory cannot account for such effects. Performance must be taken into account in the description of child phonology data.