Psychometrics is the science of psychological measurement. For an intelligence test to be considered valid, it must be evaluated using the principles and methods of psychometrics. In this chapter we introduce basic psychometric techniques such as factor analysis and review the evolution of psychometric theories of intelligence, with emphasis on the Cattell-Horn-Carroll theory of cognitive abilities.

People have been appraising each other’s capacities for a long time, at least informally. In any society, knowing who is clever, quick-witted, and knowledgeable – and who is not – confers significant advantages as we cultivate friendships, build alliances, compete with rivals, and contend with enemies. In everyday interactions, our appraisals need only be approximately correct rather than rigorously precise. For the same reasons we do not need formal measurements to decide who is trustworthy, loyal, helpful, friendly, courteous, or kind, we do not need intelligence tests – tests of people’s cognitive abilities – to know which of our acquaintances are bright. We can see their intelligence easily enough in their vocabulary, in their humor, in their creativity, and in their achievements (Borkenau & Liebler, 1995; Borkenau, Mauer, Riemann, Spinath, & Angleitner, 2004; J. Kaufman, 2016; Murphy, Hall, & Colvin, 2003). Such outward signs of intellect allow us to judge quickly and with reasonable accuracy if someone is about average in intellect, well above average, or well below. If our appraisals prove to be grossly inaccurate, we can update them as needed.

Two fields that have embraced controversy from their inception are intelligence testing and specific learning disabilities. This book, with its focus on the assessment of specific learning disabilities (SLDs), unites the two, providing a double dose of controversy. Indeed, one does not need to look very far to understand that professionals differ dramatically in their perceptions about SLDs — causes, definitions, diagnosis, treatment — and about the role that should be played by IQ tests and other standardized neuropsychological tests during the diagnostic process.

Shepherd's insightful history lesson in Chapter 1, rooted in neurology and special education and always oriented toward a pragmatic, educational perspective, delineates the different roads that have been traveled by the pioneers in the field, and enlightens us as to where these paths have led in the present. Yet other histories of learning disabilities also abound in this book, notably Spreen's neurological history (Chapter 9) that emphasizes theory and research, and stresses SLD subtypes; Reitan and Wolfson's history of SLDs from a neuropsychological research perspective (Chapter 10), in which the emphasis is on discriminating SLD individuals from normal and brain-injured individuals, and on understanding each SLD person's unique neuropsychological profile; and Mather and Woodcock's approach (Chapter 3) that cites the historical precedents for interpreting SLDs in terms of ‘the common theme … that learning disabilities arise from a deficiency in basic cognitive processes, which in turn contribute to academic failure’ (see p. 77).

This book, Specific learning disabilities and difficulties in children and adolescents: Psychological assessment and evaluation, is devoted to the topic of specific learning disabilities, with a focus on their assessment. The terms ‘learning disabilities’ (LD) and ‘specific learning disabilities’ (SLD) are used interchangeably throughout the book. When a chapter author uses the term learning disabilities, it should always be interpreted as specific learning disabilities.

The field of learning disabilities assessment has long been controversial, and remains so as the twenty-first century begins. Controversies have raged over definitions, interventions, and instruments. Recent articles in the Journal of Learning Disabilities (by leaders in the field such as Linda Siegel, Keith Stanovich, and Frank Vellutino) have called for the elimination of the IQ—achievement discrepancy from the definition of specific learning disabilities, and some leaders are demanding the elimination of IQ tests from the psychoeducational assessment process. To some extent, these arguments are based on the continued use of Wechsler's scales for the diagnosis of specific learning disabilities. We deal with these controversies in the concluding chapter of this book. However, one of the reasons that we wanted to write this book in the first place was because of the many alternatives to the Wechsler scales that became available during the last two decades of the previous millennium.

HISTORY OF INTELLIGENCE TESTING

Intelligence tests were conceived in a theoretical void and born into a theoretical vacuum. During the last half of the 19th century, first Sir Francis Galton in England (1869, 1883) and then Alfred Binet in France (Binet & Henri, 1895) took turns in developing the leading intelligence tests of the day. Galton, who was interested in men of genius and in eugenics, developed his test from a vague, simplistic theory that people take in information through their senses, and thus the most intelligent people must have the best developed senses. His test included a series of sensory, motor, and reaction-time tasks, all of which produced reliable, consistent results (Galton, the half-cousin of Charles Darwin, was strictly a scientist, and accuracy was essential) but none of them proved to be valid as measures of the construct of intelligence (Cohen, Montague, Nathanson, & Swerdlik, 1988). Galton developed a statistic that was the forerunner of the coefficient of correlation, and that proved to be his own undoing; shortly after his friend and biographer Karl Pearson perfected the statistic, a few studies revealed that the Galton intelligence test was misnamed because it did not correlate meaningfully with pertinent cognitive variables (such as grade point average).

In fact, the research studies that sounded the death knell for Galton's test conducted in the United States around the turn of the century at Galton-inspired laboratories at Columbia and Cornell were as flawed as the test itself (e.g., very small sample size, highly restricted sample of bright subjects).