An EM approach to parameter estimation for the Zinnes and Griggs paired comparison IRT model

Borman et al. recently proposed a computer adaptive performance appraisal system called CARS II that utilizes paired comparison judgments of behavioral stimuli. To implement this approach, the paired comparison ideal point model developed by Zinnes and Griggs was selected. In this article, the authors describe item response and information functions for the Zinnes and Griggs model and present procedures for estimating stimulus and person parameters. Monte carlo simulations were conducted to assess the accuracy of the parameter estimation procedures. The results indicated that at least 400 ratees (i.e., ratings) are required to obtain reasonably accurate estimates of the stimulus parameters and their standard errors. In addition, latent trait estimation improves as test length increases. The implications of these results for test construction are also discussed.