A new index of item discrimination power (IDP), dimension-corrected Somers' D (D2) is proposed. Somers' D is one of the superior alternatives for item-total- (Rit) and item-rest correlation (Rir) in reflecting the real IDP with items with scales 0/1 and 0/1/2, that is, up to three categories. D also reaches the extreme value +1 and -1 correctly while Rit and Rir cannot reach the ultimate values in the real-life testing settings. However, when the item has four categories or more, Somers' D underestimates IDP more than Pearson correlation. A simple correction to Somers' D in the polytomous case seems to lead to be effective in item analysis settings. In the simulation with real-life items, D2 showed very few cases of obvious underestimation and practically no cases of obvious overestimation. With certain restrictions discussed in the article, D2 seems to be a good alternative for these classic estimators not only with dichotomous items but also with the polytomous ones. In general, the magnitudes of the estimates by D2 are higher than those by Rit, Rir, and polychoric correlation and they seem to be close of those of bi- and polyserial correlation coefficients without out-of-range values.