Central Range Nomograph

The person sample used for test construction and validation does not have the same distribution as the people to whom you plan to administer the test. The sample may be spread out - giving a high separation, or shrunk together - giving a low one. What will be the person separation (and reliability) for you? The "Central Range" nomograph (overleaf) can be helpful. Think about your target group. What logit range would encompass the central 50% of their distribution? Or 67%? Or 90%? For example, you may think that a range of 3 logits would include the central 50% of your group. On the left side of the nomograph, locate this central range on the x-axis, 3 logits. Go up until you meet the diagonal line corresponding to the % of the group, 50%. The left-hand y-axis shows that this is the same as specifying that the S.D. of your group is 2.2 logits. Then proceed to the right-hand side of the nomograph. On the x-axis, locate the average S.E. of person measures from the sample data, e.g., .31 logits. This is usually good enough for your group. Go up from the S.E. until you meet the horizontal line drawn from the left-side. Proceed up the nearest diagonal to the expected separation (and reliability). Here an encouraging 7 (and .98).
J. Linacre

Linacre J.M. (1998) Central Range Nomograph. Rasch Measurement Transactions 11:4 p. 603-4.

Central Range Nomograph. Linacre J.M. … Rasch Measurement Transactions, 1998, 11:4 p. 603-4.

Rasch Books and Publications
Invariant Measurement: Using Rasch Models in the Social, Behavioral, and Health Sciences, 2nd Edn. George Engelhard, Jr. & Jue Wang	Applying the Rasch Model (Winsteps, Facets) 4th Ed., Bond, Yan, Heene	Advances in Rasch Analyses in the Human Sciences (Winsteps, Facets) 1st Ed., Boone, Staver	Advances in Applications of Rasch Measurement in Science Education, X. Liu & W. J. Boone	Rasch Analysis in the Human Sciences (Winsteps) Boone, Staver, Yale
Introduction to Many-Facet Rasch Measurement (Facets), Thomas Eckes	Statistical Analyses for Language Testers (Facets), Rita Green	Invariant Measurement with Raters and Rating Scales: Rasch Models for Rater-Mediated Assessments (Facets), George Engelhard, Jr. & Stefanie Wind	Aplicação do Modelo de Rasch (Português), de Bond, Trevor G., Fox, Christine M	Appliquer le modèle de Rasch: Défis et pistes de solution (Winsteps) E. Dionne, S. Béland
Exploring Rating Scale Functioning for Survey Research (R, Facets), Stefanie Wind	Rasch Measurement: Applications, Khine	Winsteps Tutorials - free Facets Tutorials - free	Many-Facet Rasch Measurement (Facets) - free, J.M. Linacre	Fairness, Justice and Language Assessment (Winsteps, Facets), McNamara, Knoch, Fan
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Rasch Measurement Transactions & Rasch Measurement research papers - free	An Introduction to the Rasch Model with Examples in R (eRm, etc.), Debelak, Strobl, Zeigenfuse	Rasch Measurement Theory Analysis in R, Wind, Hua	Applying the Rasch Model in Social Sciences Using R, Lamprianou	El modelo métrico de Rasch: Fundamentación, implementación e interpretación de la medida en ciencias sociales (Spanish Edition), Manuel González-Montesinos M.
Rasch Models: Foundations, Recent Developments, and Applications, Fischer & Molenaar	Probabilistic Models for Some Intelligence and Attainment Tests, Georg Rasch	Rasch Models for Measurement, David Andrich	Constructing Measures, Mark Wilson	Best Test Design - free, Wright & Stone Rating Scale Analysis - free, Wright & Masters
Virtual Standard Setting: Setting Cut Scores, Charalambos Kollias	Diseño de Mejores Pruebas - free, Spanish Best Test Design	A Course in Rasch Measurement Theory, Andrich, Marais	Rasch Models in Health, Christensen, Kreiner, Mesba	Multivariate and Mixture Distribution Rasch Models, von Davier, Carstensen

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