Browsing by Author "Dunbar, Stephen B."

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    Standard errors of correlations adjusted for incidental selection
    (1990) Allen, Nancy L.; Dunbar, Stephen B.
    The standard error of correlations that have been adjusted for selection with commonly used formulas developed by Pearson (1903) was investigated. The major purposes of the study were (1) to provide large-sample approximations of the standard error of a correlation adjusted using the Pearson-Lawley three-variable correction formula; (2) to examine the standard errors of adjusted correlations under specific conditions; and (3) to compare various estimates of the standard errors under direct and indirect selection. Two theory-based large-sample estimates of the standard error of a correlation adjusted for indirect selection were developed using the delta method. These two estimates were compared to one another, to a bootstrap estimate, and to an empirical standard deviation of a series of adjusted correlations generated in a simulation study. The simulation study manipulated factors defined by sample size, selection ratio, underlying population distribution, and population correlations in situations that satisfied the basic assumptions of the Pearson-Lawley procedures. The results indicated that the large-sample and bootstrap estimates were very similar when the sample size was 500 and, in most cases, the simpler of the two large-sample approximations appears to offer a reasonable estimate of the standard error of an adjusted correlation without resorting to complex, computer-intensive approaches. Index terms: correlation coefficients, missing data, Pearson-Lawley corrections, selection, standard errors of correlations, validity studies.
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    Validity generalization and situational specificity: An analysis of the prediction of first-year grades in law school
    (1981) Linn, Robert L.; Harnisch, Delwyn L.; Dunbar, Stephen B.
    Results from 726 validity studies were analyzed to determine the degree of validity generalization of the Law School Admission Test for predicting first-year grades in law school. Four validity generalization procedures were used and their results compared. As much as 70% of the variance in observed validity coefficients could be accounted for by differences in the within-study variability of LSAT scores, simple sampling error, and between-study differences in criterion reliability. The 90% credibility value for the true validities was estimated to be .45, and the average true validity was estimated to be .54. Despite the substantial degree of validity generalization, law school and the year the study was conducted explained significant portions of the residual variance in validities. Thus, some degree of situational specificity of validity remained.