Abstract: Reliability is a very important idea in classical test theory. Under classi-cal true score model reliability coefficient R=D(T)/D(x). In general, it isconsidered that there exists no relation between reliability and item responsetheory. But it is interesting to notice that reliability coefficient is just thesame as η_X~2θ=1-MD(x/θ)/D(x), which is nonlinear correlation ratio be-tween the examinee's observational scores and latent trait scores. A new me-thod of estimating reliability was obtained on the basis of the relation. Therelationship between coefficient of correlation and correlation ratio between xand θ are discussed.

Key words: reliability coefficient, correlation ratio, latent trait, classical test theory, item response theory, reliability estimating