Important Principles of Measurement Made Explicit

Chapter 2 of ARM 4th Ed is entitled Important Principles of Measurement Made Explicit. The chapter introduces the two based components of the Rasch model: person ability and item difficulty by using the analogy of a high jump competition at an athletics meet. The probabilistic nature of success and failure at each jump height is canvassed. The data for Chapter 2 are taken from the records of a children’s classroom mathematics test generated by a group of primary school teachers. The method of calculating the odds of success: failure is demonstrated and the logarithmic transformation of those odds is shown to be the base of the Rasch unit of measurement—the logit (from log odds units).