Sampling and generalisability in developmental research: Comparison of random and convenience samples of older adults

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Abstract
Research in the developmental sciences is based largely on samples of convenience rather than samples drawn at random from the population. The important question of whether results observed in samples of convenience generalise to the larger population has not been studied directly. Because of demographic growth in the proportion of older adults in the population and increases in diversity across the lifespan, it is especially important to address this issue in aging adults. We compared the performance of older adults (65–100 years) on demographic and psychological measures for a random sample of community dwelling adults and two samples of convenience. Significant differences were observed on less than half the variables. When differences were present, participants in the convenience samples were advantaged compared to participants from the random sample. Differences were larger in some domains than others but remained small to moderate in magnitude. There were minimal differences in between-person variability and patterns of correlations among variables between the convenience and random samples. Results indicate the need for additional studies contrasting random and convenience samples to explore the parameters of external validity in psychological aging research.