Jack McArdle and Kelly Kadlec, January 2007
We use the term NGCS as a shorthand way to refer to the US National Growth and Change Studies. This is not a single study, but rather a program of research started at the University of Denver in 1978 by Jack McArdle and John Horn, and we are now located in the NGCS Laboratory at the University of Southern California in Los Angeles. Our main substantive interest was to use all available collections of psychological tests to better describe and understand the many changes that seem to occur to people over the adult age life-span (18-95) To date, our NGCS research has been funded by the National Institute on Aging (NIA) and we have probed deepest into the age-related growth and declines of adult intellectual functioning.
The first research project conducted by NGCS was a large scale collection of data on the well-known and widely used Wechsler Adult Intelligence Scales (WAIS). This involved the collation of all existing sets of WAIS data, and the current archive includes over 100,000 individual protocols. The analyses of the first study asked the questions such as, "What does the WAIS measure?" and "How can the WAIS be improved?" This collection also provided longitudinal data from ten previous research studies, and this "mega-collection" led to many new developments in longitudinal methods.
A second study carried out by the NGCS was a followup of data collection from members of the well known Bradway Longitudinal Study. These 111 individuals had been measured five times before, at average ages of 4, 14, 30, 42 (see Bradway & Thompson, 1962; Kangas & Bradway, 1971), and again by the McArdle in 1984 at ages 55-57. In our first wave of new data collection (1992-93) we obtained a sixth wave of data at approximately 64 years of age on N=51 of the original Bradway longitudinal participants. This new data collection of the Bradway participants was opportune due to the high mortality rate. However, the time between ages 55 and 70 seems to be an inflection point in the growth process of many cognitive variables, so intensive measurement at these ages is needed for accurate estimation of individual differences in declining cognitive functions. This led us to initiate a third collection of new data based on a repeated (fourth) adult testing of the well-known Berkeley Growth Study participants (Aged ∼ 72, with N=46 still available). The resulting Bradway-Berkeley combination (now called the Bradway-McArdle Study), when considered as a single longitudinal data set, is one of the largest collections of life-span (ages 5-75) cognitive information in any of the world's archives
The third project conducted by NGCS staff was a continuation of the Woodcock-Johnson (WJ) norming study. The WJ-R scales are a wide-range comprehensive set of individually administered tests of intellectual ability, scholastic aptitude, and achievement. Between 1988 and 1996 the NGCS Laboratory collected longitudinal retest information on persons who had previously been measured as part of the norming sample of the Wookcock-Johnson Revised (WJ-R) tests (see McGrew, et al, 1991). This study is based on data from a sample of about 1,200 participants who provided WJ-R data on at least two occasions of measurement (for up to 17 WJ-R tests) and includes persons of ages 2 to 95 (average age = 20.3). Some of these people participated in a relatively short-term test-retest (e.g., less than one year) and other participated in longer term test-retest (about 2 years). In this second study we asked some basic questions, such as: "How do broad cognitive functions grow and change within an individual over age?" and "Are these growth and change patterns different from one variable to another?" The longitudinal data from these studies have been combined and are being analyzed by NGCS researchers.