Likert or Rasch? Nothing is more applicable than good theory.

In nursing research many concepts are measured by questionnaires. Respondents are asked to respond to a set of related statements or questions. In unidimensional scaling these statements or questions are indicants of the same concept. Scaling means to assign numbers to respondents, according to their position on the continuum underlying the concept. It is very common to use the summative Likert scaling procedure. The sumscore of the responses to the items is the estimator of the position of the patient on the continuum. The rationale behind this procedure is classical test theory. The main assumption in this theory is that all items are parallel instruments. The Rasch model offers an alternative scaling procedure. With Rasch both respondents and items are scaled on the same continuum. Whereas in Likert scaling all items have the same weight in the summating procedure, in the Rasch model items are differentiated from each other by 'difficulty'. The model holds that the probability of a positive response to an item is dependent on the difference between the difficulty of the item and the value of the person on the latent trait. The rationale behind this procedure is item response theory. In this paper both scaling procedures and their rationales are discussed.