Slosson and Peabody IQS of Mentally Retarded Adults

Abstract

Though standardized on children, the Peabody Picture Vocabulary Test and Slosson Intell~gence Test might be useful briefly to assess intelligence of mentally retarded adults. Reasonable validity coefficients for WAIS and Peabody ( r = .61 to .74, mean = .70) have been reported ( 3 ) as well as mean Peabody IQs which overestimated WAIS Full Scale 1Qs by only 2 to 6 points. With respect to WAIS-Slosson comparisons, one validity coefficient was .81 ( l ) , but the mean Slosson IQ of their mildly retarded community adults underestimated the WAIS IQ by nearly 14 points. In another study ( 2 ) the mean Slosson IQ underestimated the WAIS IQ of mildly retarded adult offenders by only 4 points. Since comparisons of Peabody and Slosson IQs of mentally retarded adults have not been made yet, Peabody IQs were obtained from the 15 participants of a previous study (1) and compared to their respective Slosson and WAIS Full Scale IQs. The Peabody was administered within 3 wk. of the latter tests. Subjects resided in community facilities, had a mean chronological age of 41 yr. (range = 25 to 54 ) , and obtained a mean WAIS Full Scale IQ of 59 (range = 50 to 69 ) . A repeated measures analysis of variance indicated significant over-all mean differences (F2.= = 33.28, p < .001). ScheffC's post hoc multiple comparisons showed significant differences between the mean WAIS Full Scale IQ of 59.3 (SD. 7.03) and Slosson IQ of 45.6 (SD, 7.23) but not the mean Peabody IQ of 63.9 (SD, 10.06). Mean Slosson and Peabody IQs not only differed significantly, but the respective individual difference scores ranged from Slosson IQs which were 2 points higher to 39 points lower (Mdn = -18). The two tests correlated only 3 1 ; the Peabody-WAIS validity coefficient of .35 was much lower than the Slosson-WAIS value of .81. Even though mean Peabody and WAIS IQs were reasonably close, individual difference scores varied widely, from Peabody IQs that were 20 points lower to 15 points higher (Mdn = -4). This contrasted with the Slosson IQ which was always lower than the WAIS IQ; difference scores ranged from 6 to 20 points lower (Mdn = -13). Even though the mean Peabody IQ may be more similar to the mean WAIS IQ than is the mean Slosson IQ, the larger inconsistent differences in individual scores suggests estimating individual WAIS IQs from the Peabody is more tenuous.' Although the present results suggest that individual scores obtained with either test be reported with caution. the small sample size urges confirmation through replication and cross-validation.