LIKERT OR RASCH - NOTHING IS MORE APPLICABLE THAN GOOD THEORY

In nursing research many concepts are measured by questionnaires. Resp ondents are asked to respond to a set of related statements or questio ns. In unidimensional scaling these statements or questions are indica nts of the same concept. Scaling means to assign numbers to respondent s, 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 positi on of the patient on the continuum. The rationale behind this procedur e is classical test theory. The main assumption in this theory is that all items are parallel instruments. The Rasch model offers an alterna tive scaling procedure. With Rasch both respondents. and items are sca led on the same continuum. Whereas in Likert scaling all items have th e 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 per son on the latent trait. The rationale behind this procedure is item r esponse theory. In this paper both scaling procedures and their ration ales are discussed.