Abstract: Test reliability of a longitudinal study is an important index in evaluating the quality of the longitudinal study. Conventional methods that estimate test reliability are not appropriate to estimate the test reliability of a longitudinal study. In recent years, four methods for estimating the test reliability of a longitudinal study were proposed. rw and r(Sw) coefficients were designed to estimate the test reliability of a longitudinal study at each wave, whereas RT and RL were designed to estimate the test reliability of the whole longitudinal study. The mathematical models and assumptions of the four reliability coefficients were briefed and their merits and demerits were pointed out in this article. RT and RL were recommended because they can be used to estimate the test reliability of a longitudinal study both at each wave and as the whole under a few assumptions.

Key words: longitudinal study, reliability, RT coefficient, RL coefficient