Maximizing rating scale information

Rating scale categories should be well defined, mutually exclusive, univocal and exhaustive (J.P. Guilford), but this is not enough. They must also be clearly differentiated in the behavior of the respondents, otherwise more categories do not mean more information. Here is an apparently well constructed scale:

How much do you fear...
1. Not at all
2. A little
3. Somewhat
4. More
5. A lot

But could the respondents clearly discriminate 5 levels of fear? The plot shows the statistical separation of the respondents obtained when adjacent categories are clumped together. Person separation is best with the categories grouped into 3 levels, and one of the dichotomizations is almost as good as the original five category scale. The choice of scoring model can dramatically alter results. The statistically best scoring model for these data was:

How much do you fear...
1. Not at all
2. A little
3. Somewhat, More, A lot

Mark Stone & Ben Wright

Maximizing rating scale information. Stone MH, Wright BD. … Rasch Measurement Transactions, 1994, 8:3 p.386

Rasch Publications
Rasch Measurement Transactions (free, online)	Rasch Measurement research papers (free, online)	Probabilistic Models for Some Intelligence and Attainment Tests, Georg Rasch	Applying the Rasch Model 3rd. Ed., Bond & Fox	Best Test Design, Wright & Stone
Rating Scale Analysis, Wright & Masters	Introduction to Rasch Measurement, E. Smith & R. Smith	Introduction to Many-Facet Rasch Measurement, Thomas Eckes	Invariant Measurement: Using Rasch Models in the Social, Behavioral, and Health Sciences, George Engelhard, Jr.	Statistical Analyses for Language Testers, Rita Green
Rasch Models: Foundations, Recent Developments, and Applications, Fischer & Molenaar	Journal of Applied Measurement	Rasch models for measurement, David Andrich	Constructing Measures, Mark Wilson	Rasch Analysis in the Human Sciences, Boone, Stave, Yale
in Spanish:	Análisis de Rasch para todos, Agustín Tristán		Mediciones, Posicionamientos y Diagnósticos Competitivos, Juan Ramón Oreja Rodríguez

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