The efficacy of teaching sounds in developmental sequence as defined by age norms was evaluated in two independent investigations. Study I was a within-subject evaluation using an alternating treatments design, with three children each receiving treatment on one early-acquired and one later-acquired phoneme relative to chronological age. Study II was an across-subject evaluation involving six children in a staggered multiple baseline paradigm, whereby three subjects were each taught one early-acquired sound and three other subjects were taught one later-acquired sound relative to chronological age. Phonological change was measured on probes of sounds excluded from each child's phonemic inventory.

General results indicated that: (a) quantitatively, change in treated phonemes and manner classes was equivocal following treatment of early-acquired and later-acquired phonemes; (b) qualitatively, the onset of change was immediate following treatment of later-acquired phonemes, but delayed following treatment of early-acquired phonemes; and (c) treatment of later-acquired phonemes led to system-wide changes in untreated sound classes, whereas treatment of early-acquired phonemes did not. These findings were considered relative to clinical intervention and theories of phonological acquisition.