J . ment. Defic. Res. (1977) 21 , 289 289

A COMPARISON OF THREE INTELLIGENCE TESTS FORTHE ASSESSMENT OF MENTAL RETARDATION

A. TRIVEDI

Woodbridge State School, Woodbridge, New Jersey 07095, U.S.A.

INTRODUCTIONMental retardation has been defined by the Ameriean Association on Mental

Deficiency (Grossman, 1973) as "significantly subaverage intellectual functioningexisting concurrently with deficits in adaptive behaviour, and manifested duringthe developmental period (p. 11)". The determination of subaverage intellectualfunctioning should be based on reliably standardised individually administered testssueh as the Binet scales or the Wechsler scales rather than a group test or one of the"quick" tests. However, some tests are available that permit the screening of in-dividuals for the purpose of determining whether further intensified testing shouldoccur. Two of these tests are to be considered here. They are the Slosson IntelligenceTest (SIT) and the Peabody Picture Vocabulary Test (PPVT).

According to Slosson (1963), the SIT is an individual screening instrument tobe used for both children and adults in evaluating mental ability. This test is an agescale ranging from 0.05 months to 24.0 years. The manual contains, in minute detail,directions for administering each item to a variety of populations (infants, handi-capped, blind, emotionally disturbed, culturally deprived and mentally retarded).The results are expressed as an MA and an IQ_. The MA is obtained by giving one-half, one, two and three months credit for each item passed, as indicated on the faceof the score sheet. The IQ, is the usual MA/CA with the stipulation that CA neverexceeds sixteen years or 192 months.

At one year the tasks begin to involve vocabulary, locomotion and self-careitems. From two years to four years the test follows the Rev. S-B scale items incontent: for example, copying a circle, making size comparisons, repeating shortsentences and constructing analogies. Later year items introduce arithmetic, simi-larities and differences, digit span and reasoning, skills and abilities that appear inthe verbal type of intelligence test. Allen and Allen (1972) write that the mostcompelling recommendation for the SIT is its rapid screening potential.

This test is designed to be a short form test and is closely related to the Stanford-Binet. The purpose of this test, according to Slosson (1963), is:

"This short intelligence test (SIT) has proved to be useful as an individualscreening instrument for both children and adults. The test has been made forthe use of school teachers, prineipals, psychometrists, psychologists, guidancecounsellors, social workers, school nurses and other responsible persons who, intheir professional work, often need to evaluate an individual's mental ability(p. 111)."A study carried out by Carlisle, Shinedling, and Weaver (1970), concerning

Received 18th January, 1977

290 THREE INTELLIGENCE TESTS

the use of the Slosson Intelligence Test (SIT) with mentally retarded residents,indicates a correlation of .91 between scores on the SIT and the Kuhlmann Extensionof the Binet Test and suggests that the SIT is a useful test to use with retardedpersons.

Carlisle et al. (1970) presented the following description of the test:"The test yields both an MA and an I Q . The IQ,is the usual MA/CA althoughthe CA never exceeds 16 years. The test begins with developmentally orienteditems and proceeds to abstract definitions and arithmetic problems. Items up tothe 8 year level center around tasks and concepts with which more retardedcome in contact and, hence, have the opportunity to become familiar. This isless true for items from the 8 year to 9 year level. From the 9 year to the 27 yearlevel, the majority of the items are arithmetic or definitions type. Personsfunctioning on a level of mild retardation who reside in an institution wouldpossibly be penalized, having had less contact with these types of problems thanindividuals in a non-institutional setting. The test takes from 10 to 15 minutesto give and score. Less training is required for the admission of this test than theStanford-Binet (p. 865)."DeLapa (1968) reports that the Slosson Intelligence Test is so nearly like the

Stanford-Binet that, for practical purposes, it can be thought of as a short form of theStanford-Binet. Though validity data on the SIT vary in magnitude, they are notsignificantly higher than those on the PPVT. Slosson reported the results of a cor-relation study comparing SIT and Stanford-Binet scores on 701 of the standardranking sample separate coefficients were computed for each CA group from fourto sixteen years. These coefficients ranged from .90 to .98. Other studies havesupported these findings. Both Ciacinto (1967) and DeLapa (1968) found coefficientsof .90 between SIT and Stanford-Binet IQscores on regular class students. Referenceshave already been made that Carlisle et al. (1970) reported a coefficient of .91 betweenthe two tests, based on a sample of 122 institutionalised mentally retarded subjects.Jongeward (1969), with a sample of thirty educable mentally retarded children,ages seven-five to nine-eleven, found a correlation of .80 for MA scores and .76 forIQ, scores. On the other hand, Houston and Otto (1968) reported significant butlower correlations.

Dunn (1965) states that the PPVT was designed to provide an estimate of aperson's verbal intelligence through measuring his hearing vocabulary. According toDunn (1965) the Peabody Picture Vocabulary Test is designed to provide an estimateof a subject's verbal intelligence through measuring his hearing vocabulary. Thetest has wide utility as a chnical tool. Besides being effective with average subjects,it has special value with mental retardates. Since subjects are not required to read,the scale is especially fair for them and since responses are non-oral, it is madeappropriate for the speech impaired (especially the expressive aphasic and stutterer),and has advantages with certain autistic, withdrawn and psychotic persons byreducing the tensions of the testing situation. The PPVT has a number of advantages.Among these are the following: (1) the test has high interest value and, therefore,is a good rapport establisher; (2) extensive specialised preparation is not needed for

A. TRIVEDI 291

its administration; (3) it is quickly given in 10 to 15 minutes; (4) scoring is com-pletely objective and quickly accomplished in a few minutes; (5) no oral response isrequired; (6) alternate forms of the test are provided to facilitate repeated measures;(7) the test covers a wide range (between two years six months and eighteen years).

Kaufman and Ivanoff (1969) evaluated the mentally retarded with the PeabodyPicture Vocabulary Test (PPVT). The purpose of their study was to investigate thesuitability of using the Peabody Picture Vocabulary Test (PPVT) in a rehabilitationcentre with mentally retarded clients in comparing the PPVT with the WeehslerAdult Intelligence Scale (WAIS) and the reading section of the Wide Range Achieve-ment Test (WRAT). These investigators concluded that although the PPVT mayprovide an adequate screening instrument with some populations, in working withthe mentally retarded the reading section on the WRAT measures functional abilitycomparable to the WAIS full scale IQscore. However, where the PPVT is used withthe mentally retarded, it was suggested by these investigators that PPVT mentalages be substituted for PPVT I Q scores.

Rice and Brown (1967) questioned the feasibility of using the PPVT as a reliableindicator of intellectual functioning for educable mental retardates. They state thatcorrelations based on MA scores results in spuriously high coefficients, due to variancecontributed by uncontrolled CA. However, McArthur and Wakefield (1968)generalised from their findings in their study under reference that there is the con-sistent correlations between PPVT-A I Q versus SB I Q , PPVT-A I Q versus WISCfull scale I Q a n d PPVT-A I Q versus WISC verbal I Q , as shown in Table 1.

Date

MA

Correlations

PPVT-APPVT-APPVT-APPVT-APPVT-A

Table 1between Intelligence Tests^

Tests

SB-LMSB-LMWISC—FullWISC—VerbalWISC—Performance

Reliab

.85

.73

.71

.73

.55

'Adapted from McArthur and Wakefield (1968).

Shaw, Mathews, and Klove (1966) found correlations between the PPVT andthe three WISC scales significant. Rice and Brown (1967) have reported a long-termtest, retest reliability coefficient of .60, the interval being ten months, and validitycoefficients with the WISC and Stanford-Binet individual intelligence scales of .40with Form A and .54 with Form B.

With the aim of determining if the PPVT could appropriately be substitutedfor the WISC, as an individual test of intelligence, Hughes and Lessler (1965)administered these tests to one hundred and thirty-seven Negro and White, culturallydeprived, children who were suspected of being mentally retarded. They found thatthe WISC and the PPVT were generally positively correlated, but with predictable

292 THREE INTELLICENCE TESTS

race and sex differences occurring, and concluded that even though standard errorof estimates were relatively large, the PPVT could be substituted for the WISC as ascreening instrument for use in the school or by mental health clinic personnel.

However, a study carried out by Burnett (1965) in regard to comparison ofPPVT, WISC and Stanford-Binet on educable retardates shows that there existmedium relationships between the PPVT, WISC and S-B while other inter-con-nections were substantial and PPVT mean IQs were found to be significantly higherthan those of the WISC and S-B. Comparing the WISC and PPVT with emotionallydisturbed children, Himelstein and Herndon (1962) reported a correlation of .63between the full-scale WISC and PPVT.

Other studies have compared performance on the PPVT and the WeehslerAdult Intelligence Scale (WAIS). Bonner and Belden (1970) tested Negro highschool seniors and Tobias and Gorelick (1961) tested retarded adults on these twotests. The PPVT underestimated WIAS IQs. More recently, Covin and Covin (1976)found that the PPVT was more related to the WAIS Verbal and Full Scales than tothe WAIS Performance Scale and the PPVT I Q scores were generally lower. Theevidence, thus far, suggests that the PPVT-WISC relationship may be more unstablethan the SIT-WISC relationship and that the PPVT generally produces higher IQsthan the WISC. This conclusion is based on the relative lack of information comparingthe SIT and the WISC and the paucity of studies investigating the performance ofretarded persons on multiple test of intelligence seeking information as to the inter-reliability and comparable mean performance on the tests. Both the SIT and PPVThave been used appropriately as screening devices. Our concern here is with itsuse and the reliability of each as a screening device when compared with the WeehslerIntelligence Scale for Children (WISC). The purposes of this investigation were tocompare mentally retarded individuals on the SIT, PPVT, and WISC measures ofintelligence and determine the reliability among these three tests as to MA and I Qperformance.

METHOD AND PROCEDUREThirty-six students, eighteen males and eighteen females, were selected from the

resident population of a state institution for the mentally retarded with the restrictionthat their chronological ages (CA) be between thirteen and sixteen years, their IQsbetween 40 and 84, as determined by a recent administration of the WISC; andthey display no gross motor dysfunctioning. The group had a mean CA of 15.29 years(.64), a mean mental age (MA) of 8.97 years (1.63), and a mean IQof 59.6 (9.85).Standard deviations are given in parentheses. The males and females did not differon the CA, MA or I Q parameters.

The students were divided into two groups with nine males and nine females ineach group. The resulting age and I Q parameters for one group were: mean CA =15.29 years (.61), mean MA = 9.09 years (1.25), and mean I Q = 59.6 (8.4). Forthe other group they were: mean CA = 15.29 years (.68), mean MA = 9 . 1 2 years(1.76), and mean I Q = 59.6 (H. l ) . There were no sex differences in either groupon any of these parameters.

A. TRIVEDI 293

The PPVT and SIT were administered in one session to each individual incounterbalanced order. The tests were administered so that sex and order of presenta-tion of tests were orthogonally balanced. Although the experimenter had selected thethirty.-six students from a larger pool of students and also administered two tests,testing did not occur until two weeks after the selection of the students after they hadbeen assigned to their respective conditions. Information on each individual's per-formance on the WISC was not reviewed at time of testing.

RESULTSTable 2 presents the means and SDs for each sex and order of presentation for

MA and I Q o n the WISC, PPVT, and SIT scores.

Two three-way analyses of variance (Sex X Order of Presentation X Tests)were performed, one with I Q and the other with MA as the dependent measure, inorder to (ietermine the effect of these parameters on performance. When I Q wasthe dependent measure, the only reliable difference was the main effect of Tests(F = 85.65, 2/64, p < .001). A Tukey B test of multiple comparisons revealed thatthe mean PPVT scores (71.31) were higher than the mean WISC scores (59.56)and mean SIT scores (58.56, p < .01). When MA was the dependent measure, theonly reliable difference was in the Order of Presentation X Tests interaction {F =4.11, 2/64, p < .05). This interaction was due to the improved performance on thePPVT when this test followed administration of the SIT as compared with the lower

Table 2

Means and Standard Deviations {SD) for MA and Idon the WISC, PPVT, and SIT by Sexand Order of Presentation

MA IQ_

Mean SD Mean SD

Male

Female

Order A

Order B

WISCPPVTSIT

WISCPPVTSIT

WISCPPVTSIT

WISCSITPPVT

109.3113.9105.6

109.0107.6106.0

109.1106.3105.8

109.4108.9117.6

19.917.416.4

19.219.114.0

15.017.212.0

21.114.616.2

60.473.459.9

58.769.258.2

59.668.857.7

59.659.573.8

9.66.77.2

10.17.57.8

8.47.26.4

11.18.26.8

294 THREE INTELLIGENCE TESTS

Table 3Correlations among Intelligence Tests as a Function of Sex, MA and IQ}

(1)

(2)

(3)

(4)

(5)

WISCWISCPPVT

WISCWISCPPVT

WISCWISCPPVT

WISCWISCPPVT

WISCWISCPPVT

WISCWISCPPVT

vsvsvs

vsvsvs

vsvsvs

vsvsvs

vsvsvs

vsvsvs

PPVTSITSIT

PPVTSITSIT

MALE, IQ'

PPVTSITSIT

FEMALE, IQ'

PPVTSITSIT

MALE, MA'

PPVTSITSIT

FEMALE, MA'

PPVTSITSIT

r =r —r —

r =r =r =

r —r =r =

r =T —

r =

r =r =r =

r =r —r =

.65

.86

.67

.67

.87

.66

.60

.89

.49 (< .05)

.70

.85

.69

.61

.89

.54 (< .02)

.76

.86

.76(6)

^All correlations were significant at p < .01, except as noted.==<//= 34. ' # = 1 6 .

performance when the PPVT was administered prior to the SIT. These two analysesindicate that the PPVT produces higher IQ,scores than the WISC andSITregardlessof order of presentation and that there is a residual effect of the SIT on the PPVTMA scores. When the PPVT preceded the SIT, the MA scores for the three testswere almost identical. Correlations were then performed comparing the MAs andIQs among the three tests.

As can be seen in (1) and (2) of Table 3, all r's were significant, but comparisonsbetween the WISC and SIT have a higher relationship than other comparisons on

A. TRIVEDI 295

both IQ, and MA. The higher reliable relationship between the WISC and SIT isalso borne out when comparisons are made based on IQ_for each sex, as can be seenin (3) and (4) of that table. The males had a lower relationship between the PPVTand SIT than the females.

That the relationships between the WISC and SIT are consistently high is alsodemonstrated in (5) and (6) when MA is the parameter. Again, the males had alower relationship between the PPVT and SIT (5). Apparently, the highly significantcorrelations between the WISC and SIT are not influenced by sex or IQ,or MA. Ther's between those two tests vary from 0.85-0.89 which is remarkably stable. However,the relationships between the WISC and PPVT range from 0.60 to 0.76 and betweenthe PPVT and SIT from 0.49 to 0.76. Although all of the relationships were significant,in practice preference may be given to use of the SIT as a screening device since theWISC-SIT relationships were more reliable.

DISCUSSIONThese results replicate Burnett's (1965) finding that the PPVT produces higher

IQscores than the WISC and that the two tests are highly correlated. Although therelationship of these two tests is high, as a screening instrument the PPVT willproduce more false-negatives than the SIT. The close proximity of the mean WISCIQs and the mean SIT IQs (59.56 and 58.86 respectively) and their respective SDs(9.80 and 7.55) as well as the mean WISC MA and the mean SIT MAs (109.2 and105.8) and their respective SDs (19.55) and 15.20) suggests that the SIT is a betterindicator of performance on the WISC although the SIT is thought of as a shortform of the Stanford-Binet (DeLepa, 1968).

This is borne out further by the relationship between the WISC and SIT ascompared with the WISC and PPVT. The correlations between the WISC and SITranged from .95 to .89 whether compared on sex, MA or IQ whereas, the WISCand PPVT correlations ranged from .60 and .76 which, although demonstratingsignificant relationships, are lower and more variable.

Of interest are the SIT-PPVT comparisons. Johnson and Shinedling (1974),testing the mentally retarded, found that PPVT, MA and IQscores were higher thanSIT scores (.55 years and 14.27 IQpoints); this study produced similar results(.41 years and 13.70 IQpoints). Again, the PPVT is spuriously high in IQ. As mightbe expected, the relationship between those two tests was less stable and the rangegreater than previous comparisons (.49 to .76).

The stability of the PPVT is also in question. When the PPVT was administeredfirst the MA scores were comparable to the WISC and SIT MA scores; whenadministered second, the PPVT AlA scores increased. Thus, if administered withina battery of other tests, the PPVT could be influenced by previous tests resulting inhigher MAs. Consequently, it would be advisable when testing the mentally retardedfor purposes of screening to use the SIT rather than the PPVT for a more reliableevaluation.

As a final concern, it should be emphasised that the PPVT and SIT are teststo be used only as screening devices to determ ine whether further evaluation of a

296 THREE INTELLIGENCE TESTS

person is required. The interest in comparative performance between tests should bemainly in whether the screening test will produce false-negative errors therebyindicating no further evaluation. An error in the opposite direction, false-positive, isnot as serious. The latter error occurs when the PPVT is compared with the WAIS(Covin and Covin, 1976). It was found here that the former error occurs when thePPVT is compared with the WISC. The preference for the SIT as a screening testis that, aside from giving a better estimate of performance on the WISC and avoidingeither error, it is not as susceptible to the possible influence of other tests administeredin the same session.

SUMMARYThirty-six mentally retarded adolescents were administered the Weehsler

Intelligence Scale for Children (WISC), the Peabody Picture Vocabulary Test(PPVT), and the Slosson Intelligence Test (SIT) to determine reliability and com-parable performance among the three intelligence tests. Correlations as a functionof I Q and MA, by sex, were computed among the three tests. All were significant[p < .01), but the WISC and SIT had consistently higher relationships than anyother comparisons on both IQ, and MA. Analyses also indicated that the PPVTproduced higher I Q scores than the WISC or SIT and that MA in the PPVT wasinfluenced by the previous administration of the SIT. It was concluded that, as ascreening device, the SIT is more reliable and approximates WISC scores moreconsistently than the PPVT.

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