What Do You Mean by a Difficult Item? On the Interpretation of the Difficulty Parameter in a Rasch Model
Three versions of the Rasch model are considered: the fixed-effects model, the random-effects model with normal distribution, and the random-effects model with unspecified distribution. For each of the three, we discuss the meaning of the difficulty parameter starting each time from the corresponding likelihood and the resulting identified parameters. Because the likelihood and the identified parameters are different depending on the model, the identification of the parameter of interest is also different, with consequences for the meaning of the item difficulty. Finally, for all the three models, the item difficulties are monotonically related to the marginal probabilities of a correct response.
KeywordsLikelihood Function Marginal Probability Item Difficulty Item Parameter Difficulty Parameter
This research was partially funded by the ANILLO Project SOC1107 Statistics for Public Policy in Education from the Chilean Government.