Assessment of the quality of the scale with the Rasch model: scale reliability and validity

The aim of the article is to present Rasch model and its application to the assessment of scale validity and reliability. The method is hardly known in Poland. The problem of good quality data is important as such a data is a prerequisite of reliable results of each statistical analysis. The Rasch model provides a different approach to the assessment of the scale quality from that having been used in Poland and described among others by David Magnusson, Jerzy Brzezinski, Jaroslaw Gorniak, Andrzej Machowski, Adam Sagan. The text consists of two parts - theoretical one and practical one. The former includes methodology of the Rasch measurement whereas the latter presents its application. Although there is a big family of the Rasch measurement models this text focuses mainly on the rating scale model. This method is used to analyze rating scale data obtained from the battery of items with different intensities designed to measure one latent trait.