MEASUREMENT RESEARCH ASSOCIATES
TEST INSIGHTS
February 2009
Greetings,
 
There is frequently a question about the number of items needed to cover the content and insure candidate separation reliability.  Our simple study revealed that the quality of items is as or more important than the absolute number of items on the test to achieve satisfactory reliability.
 
Mary E. Lunz, Ph.D.
Director

 
Test Length and Test Reliability for Multiple Choice Examinations

Some believe that longer multiple choice tests tend to be more reliable because more items automatically reduce the error of measurement.  Indeed, a sufficient number of items must be included to cover the content areas tested; however, there are other factors that contribute to how efficiently a test measures and separates candidate ability. 

The indication of reliability is candidate separation reliability, calculated as [SD2 - SE2/ SD2] using Rasch logit measures.  This reliability index is appropriate because the goal of any certification examination is to distinguish between those candidates who are worthy of passing and those who are not.  The better the test items distinguish among candidate abilities, the less measurement error there is in the examination and the higher the candidate separation reliability, regardless of the number of items.  For example, deleting poorly performing items makes the test shorter, but also increases the accuracy of measurement because the items producing the most error are eliminated from the examination. 

In this simple study, we looked at test length compared to test reliability. Six different exams were included. The table shows the tests in reliability order along with the number of items in each test.  The number of items and reliability do not appear to be related. A test of 211 items had a reliability of .93 while another test of 233 items had a reliability of .78.  It appears that the quality of the items is as important, or even more important, than the absolute number of items on the test. Of course, a reasonable number of items must be included to insure accurate measurement and content coverage.

   Table of Test Length and Reliability

Exam

N of items

Reliability

Exam 1

211

.93

Exam 2

133

.91

Exam 3

193

.90

Exam 4

129

.86

Exam 5

236

.85

Exam 6

233

.78


Measurement Research Associates, Inc.
505 North Lake Shore Dr., Suite 1304
Chicago, IL  60611
Phone: (312) 822-9648     Fax: (312) 822-9650



Rasch-Related Resources: Rasch Measurement YouTube Channel
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 Journal of Applied Measurement
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
Rasch Books and Publications: Winsteps and Facets
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 Appliquer le modèle de Rasch: Défis et pistes de solution (Winsteps) E. Dionne, S. Béland
Introduction to Many-Facet Rasch Measurement (Facets), Thomas Eckes Rasch Models for Solving Measurement Problems (Facets), George Engelhard, Jr. & Jue Wang 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
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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