A simulation study was performed to determine whether a group's average percent correct in a content domain could be accurately estimated for groups taking a single test form and not the entire domain of items. Six Item Response Theory (IRT) -based domain score estimation methods were evaluated, under conditions of few items per content area per form taken, small domains, and small group sizes. The methods used item responses to a single form taken to estimate examinee or group ability; domain scores were then computed using the ability estimates and domain item characteristics. The IRT-based domain score estimates typically showed greater accuracy and greater consistency across forms taken than observed performance on the form taken. For the smallest group size and least number of items taken, the accuracy of most IRT-based estimates was questionable; however, a procedure that operates on an estimated distribution of group ability showed promise under most conditions. An appendix discusses estimating mean group ability using a latent-variable regression model. (Contains 5 tables, 12 figures, and 21 references.) (Author/SLD)