The upper and lower twenty-seven per cent rule
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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.
[1] Cyril Burt,et al. Fundamentals of Statistics. , 1948 .
[2] Frederic M. Lord,et al. The Relation of Test Score to the Trait Underlying the Test , 1952 .
[3] T. L. Kelley. The selection of upper and lower groups for the validation of test items. , 1939 .