Human Growth and Development / Unit 13 HGD 1823 worked example, 60 to 90 seconds
The Seattle Longitudinal Study
The idea
The Seattle Longitudinal Study, begun by K. Warner Schaie in nineteen fifty-six, has tested adults' abilities every seven years, adding new participants along the way; about six thousand have taken part. Because it follows the same people over time, it avoids the cohort effects that made cross-sectional studies exaggerate decline. It found that verbal memory, spatial skills, inductive reasoning and vocabulary rise into the seventies, while numerical computation and perceptual speed decline in middle and late adulthood.
In practice
A cross-sectional study comparing today's twenty-five-year-olds with today's seventy-year-olds, confusing age with the different schooling each cohort received.
Where people slip
Longitudinal designs have their own problem: people drop out, and repeated testing brings practice gains. Middle-aged adults performed better than young adults on four of the six abilities measured. Exam items ask why longitudinal and cross-sectional results differ: cohort effects are the answer.
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