A didactic discussion of a procedure for interval estimation of change in scale reliability due to revision is provided, which is developed within the framework of covariance structure modeling. The method yields ranges of plausible values for the population gain or loss in reliability of unidimensional composites, which results from deletion or addition of scale components. The obtained confidence intervals can also be used for testing conventional as well as minimum effect hypotheses about the associated increment or drop in reliability. The approach yields as a by-product point and interval estimates for reliability of any instrument version, and is illustrated with two examples. (Contains 1 table and 3 footnotes.)