This article introduces a procedure for estimating reliability in which equivalent halves of a given test are systematically created and then administered a few days apart so that transient error can be included in the error calculus. The procedure not only estimates complete reliability (taking into account both specific-factor error and transient error) but also can estimate partial reliability (taking into account only specific-factor error). Scores from 6 different measuring instruments were analyzed with the procedure. The results indicate that the magnitude of transient error in real data can range from nonexistent to very large. It follows that traditional reliability estimates, using nonstaggered procedures, are inflated to the extent that transient error is present. (Contains 5 tables and 4 footnotes.)