Psychometric evaluation of a knowledge based examination using Rasch analysis: an illustrative guide: AMEE guide no. 72.

Classical test theory has traditionally been used to carry out post-examination analysis of objective test data. It uses descriptive methods and aggregated data to help identify sources of measurement error and unreliability in a test, in order to minimise them. Item response theory (IRT), and in particular Rasch analysis, uses more complex methods to produce outputs that not only identify sources of measurement error and unreliability, but also identify the way item difficulty interacts with student ability. In this guide, a knowledge-based test is analysed by the Rasch method to demonstrate the variety of useful outputs that can be provided. IRT provides a much deeper analysis giving a range of information on the behaviour of individual test items and individual students as well as the underlying constructs being examined. Graphical displays can be used to evaluate the ease or difficulty of items across the student ability range as well as providing a visual method for judging how well the difficulty of items on a test match student ability. By displaying data in this way, problem test items are more easily identified and modified allowing medical educators to iteratively move towards the 'perfect' test in which the distribution of item difficulty is mirrored by the distribution of student ability.