Item response theory (IRT) has been used extensively to study differential item functioning (dif) and to identify potentially biased items. The use of IRT for diagnostic purposes is less prevalent and has received comparatively less attention. This study addressed differential objective function (dof) to identify potentially biased content units. IRT was used to estimate person abilities and item difficulties, which were used to compute residual objective scores. Residual objective scores were analyzed with analysis of variance using the independent variables gender and ethnicity. Data were from mathematics subtests from the 1992 Connecticut Mastery Test census administration of eighth graders and its database of approximately 32,000 Connecticut eighth graders. The examples illustrate how dof outcomes can be used to identify potentially biased content units, to provide diagnostic information at the content level, and to construct profiles of content-based performance for different demographic subgroups. Ten figures and two tables present analysis results. Two appendixes present dif statistics by demographic subgroup and item-level statistics for dof objectives in four tables. (Contains 11 references.) (Author/SLD)