Effects of Local Item Dependence on the Fit and Equating Performance of the Three-Parameter Logistic Model

Unidimensional item response theory (IRT) has be come widely used in the analysis and equating of edu cational achievement tests. If an IRT model is true, item responses must be locally independent when the trait is held constant. This paper presents several mea sures of local dependence that are used in conjunction with the three-parameter logistic model in the analysis of unidimensional and two-dimensional simulated data and in the analysis of three mathematics achievement tests at Grades 3 and 6. The measures of local depen dence (called Q 2 and Q 3 ) were useful for identifying subsets of items that were influenced by the same fac tors (simulated data) or that had similar content (real data). Item pairs with high Q 2 or Q 3 values tended to have similar item parameters, but most items with similar item parameters did not have high Q 2 or Q 3 values. Sets of locally dependent items tended to be difficult and discriminating if the items involved an accumulation of the skills involved in the easier items in the rest of the test. Locally dependent items that were independent of the other items in the test did not have unusually high or low difficulties or discrimina tions. Substantial unsystematic errors of equating were found from the equating of tests involving collections of different dimensions, but substantial systematic er rors of equating were only found when the two tests measured quite different dimensions that were presum ably taught sequentially.