Abstract
A simplified re-derivation of the formula underlying the rule is presented, followed by a derivation of the comparable rule for the unitrectangular distribution, which turns out to be a 33-per cent rule. Critical comments are offered concerning two assumptions: normality of the score distribution and equality of mean standard errors of measurement in the high and low groups.
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References
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Cureton, E.E. The upper and lower twenty-seven per cent rule. Psychometrika 22, 293–296 (1957). https://doi.org/10.1007/BF02289130
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DOI: https://doi.org/10.1007/BF02289130