A reliable prediction of the isotropic hyperfine coupling constant A(iso) is still a difficult task for ab initio calculations. In previous studies, the configuration selected multireference configuration interaction method in combination with perturbation theory to correct the wave function (MRCI/B-K) yielded accurate isotropic hyperfine coupling constants very economically. The present study gives a detailed analysis of the MRCI/B-K method based on the X(2) Pi state of CH as a test case. Furthermore, a comparison to various other methods such as Moller-Plesset perturbation theory and the coupled cluster approach is made. The success of the MRCI/B-K method in predicting isotropic hyperfine coupling constants is explained in terms of the influence of higher than double excitations.