Intellectual Disability and the Death Penalty by Marc J. Tassé Ph.D

Author:Marc J. Tassé Ph.D.
Language: eng
Format: epub
Publisher: ABC-CLIO

Assessing Intellectual Functioning

Despite the debate regarding what is the prevailing theory of human intelligence, using individually administered standardized tests of intelligence that have a comprehensive set of tasks and subtests with adequate national norms remains the recommended means of determining Prong 1 of the diagnostic criteria for intellectual disability (APA, 2013; Schalock et al., 2010). The use of the full-scale IQ score remains the best way to represent the construct of general intellectual functioning for the purpose of making a diagnosis of intellectual disability (McGrew, 2015; Schalock et al., 2010). Significant deficit in intellectual functioning is operationally defined as a standard score that is approximately two standard deviations below the population mean, with consideration of all sources of measurement error (APA, 2013; Schalock et al., 2010). We will discuss measurement error, but first we should define some of the key terms used.

Defining Terms

We will define these psychometric terms before moving any further. Intelligence quotients (IQ) are a form of standard score commonly used by tests of intellectual functioning. A person’s standard score on a test is calculated by transforming the individual’s obtained raw score on Test A (e.g., the sum of the number of correct responses on a test) using the population’s known mean and standard deviation on Test A, which transforms the individual’s test performance onto a common metric allowing us to compare his or her score to anyone else tested with Test A. Standard scores are possible only for tests where, if administered to the entire population, the distribution of all test scores on said test would be normally distributed (see Figure 1.1 in Chapter 1). A percentile score is one form of a standard score that permits the interpretation of a person’s performance in relation to a reference group. Although not a requirement, in the case of many psychological tests the scale for standard scores is set to have a mean or average score of 100 and a standard deviation of 15. Thus, a test performance that results in a standard score of 70 is said to be “significantly” below average or approximately two standard deviations below the population mean. A standard deviation is a unit of measure that indicates the distance from the average. During the standardization phase of the development of a standardized test, the test and its items are administered to a large and representative sample of the reference group of interest or population. This is generally referred to as the standardization sample or norming group. From this norming group, the test developers compute the population’s mean score and standard deviation on the test. The mean score and standard deviation are essential to transforming subsequently obtained raw scores (i.e., the sum of the number of correct items) on said test to a standard scale score (e.g., intelligence quotient, or IQ).

Tests of Intelligence

We do not have sufficient space in this chapter to review in depth all the available tests of intelligence that are adequately constructed, standardized, and normed for the purpose of evaluating intellectual functioning when making a determination of intellectual disability.

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