  Research on the test structure of the Force Concept Inventory (FCI) has
largely been performed with exploratory methods such as factor analysis and
cluster analysis. Multi-Dimensional Item Response Theory (MIRT) provides an
alternative to traditional Exploratory Factor Analysis which allows statistical
testing to identify the optimal number of factors. Application of MIRT to a
sample of $N=4,716$ FCI post-tests identified a 9-factor solution as optimal.
Additional analysis showed that a substantial part of the identified factor
structure resulted from the practice of using problem blocks and from pairs of
similar questions. Applying MIRT to a reduced set of FCI items removing blocked
items and repeated items produced a 6-factor solution; however, the factors had
little relation the general structure of Newtonian mechanics. A theoretical
model of the FCI was constructed from expert solutions and fit to the FCI by
constraining the MIRT parameter matrix to the theoretical model. Variations on
the theoretical model were then explored to identify an optimal model. The
optimal model supported the differentiation of Newton's 1st and 2nd law; of
one-dimensional and three-dimensional kinematics; and of the principle of the
addition of forces from Newton's 2nd law. The model suggested by the authors of
the FCI was also fit; the optimal MIRT model was statistically superior.
