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Abstract:
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The CNSTAT Panel recommended development of an Item Response Theory (IRT) model to measure and monitor motor carrier highway safety. In this presentation, the proposed IRT model will be described, including various refinements and modifications to the basic model. The basic model first treats the number of inspections for a given carrier and inspection type as a Poisson process which can depend on operational characteristics of the carrier. For a given inspection type there will be a number of safety-related characteristics that are assessed and determined for compliance/violation. These form the "items" of the IRT model that are modeled conditional on the number of inspections. These items are thought to arise from a number of latent constructs, and so a multidimensional IRT model is proposed. In particular, application of the bi-factor IRT model, which includes a dominant overall factor and several independent subfactors, will be described. Extensions of the model to allow for absolute versus relative ranking of the carriers and the relative merits of stratifying versus model-based adjustment for carrier characteristics will also be discussed.
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