As a global measure of precision, item response theory (IRT) estimated reliability is derived for four coefficients (Cronbach's a, Feldt-Raju, stratified a, and marginal reliability). Models with different underlying assumptions concerning test-part similarity are discussed. A detailed computational example is presented for the targeted coefficients. A comparison of the IRT model-derived coefficients is made and the impact of varying ability distributions is evaluated. The advantages of IRT-derived reliability coefficients for problems such as automated test form assembly and vertical scaling are discussed.