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Analyzing School Effects: Ancova With a Fallible Covariate1

THE analysis of variance, covariance method (ANCOVA) has been employed in nonexperimental school effects studies to control for differential input when studying the differential impact of schools as a categorical treatment factor on some output variable. One of the numerous hazards (Smith, 1957) to interpreting these ANCOVA findings results from the use of input measures known to have considerable errors of measurement. As a consequence, input may not be completely controlled, i.e., the &dquo;adjusted&dquo; treatment variance which is labeled the &dquo;differential&dquo; school effect may, 1