December 2008
After having worked with many tests over the years, the impact of measurement error on test reliability has become extremely clear.  It is also my observation that the quality, rather than the number of items, is the key to reducing measurement error and improving test reliability.
Surintorn Suanthong, Ph.D.
Manager, Test Analysis and Research
Reliability for Multiple Choice Examinations

There are several ways to calculate the reliability of multiple choice examinations. Each formula means something slightly different with regard to the precision with which candidates are measured.
KR-20, Cronbach Alpha and Rasch Separation reliability are all estimates of
"true person variance / observed person variance" for the sample. KR-20 is based on split-halves, Cronbach Alpha is based on analysis of variance. These two are identical for complete, dichotomous tests, and are based on raw scores. Rasch Separation reliability is computed from the Rasch measures and their respective standard errors. The biggest practical difference is that KR-20 or Cronbach Alpha includes extreme items that are calculated to have small error variance and may increase reliability. For Rasch candidate separation reliability, extreme item measures are estimated to have large errors associated with the measure that are likely to lower the reliability.
Regardless of the formula used, the impact of the error of measurement on the variance of the candidates' scores is the key to understanding reliability. Generally, the larger the error of measurement is, the lower the reliability.  Measurement error is affected by 1) the number of items, 2) statistical performance of the items, and 3) quality, clarity, and relevance of the items. Therefore, the number of items on the test is not the sole factor that contributes to the reliability estimate. 
The table shows results from five different multiple choice examinations. The number of items on the exam, in part relates to the error of measurement. However, the key is the amount of standard deviation relative to the error of measurement. When the standard deviation is higher relative to the error of measurement, the estimated reliability is higher, but when the error of measurement is higher relative to the standard deviation as in Exam 5, the estimated reliability is lower. The goal is to reduce the error of measurement so that differences among the candidates are accurately measured. In the end, it all depends on the quality of the items, rather than the number of items on the exam.   



Number of Items

Error of Measurement*

Standard Deviation*


Exam 1





Exam 2





Exam 3





Exam 4





Exam 5





*presented in logits

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

To be emailed about new material on
please enter your email address here:

I want to Subscribe: & click below
I want to Unsubscribe: & click below

Please set your SPAM filter to accept emails from welcomes your comments:
Please email inquiries about Rasch books to books \at/

Your email address (if you want us to reply):


FORUMRasch Measurement Forum to discuss any Rasch-related topic

Coming Rasch-related Events
Oct. 6 - Nov. 3, 2023, Fri.-Fri. On-line workshop: Rasch Measurement - Core Topics (E. Smith, Facets),
Oct. 12, 2023, Thursday 5 to 7 pm Colombian timeOn-line workshop: Deconstruyendo el concepto de validez y Discusiones sobre estimaciones de confiabilidad SICAPSI (J. Escobar, C.Pardo)
June 12 - 14, 2024, Wed.-Fri. 1st Scandinavian Applied Measurement Conference, Kristianstad University, Kristianstad, Sweden
Aug. 9 - Sept. 6, 2024, Fri.-Fri. On-line workshop: Many-Facet Rasch Measurement (E. Smith, Facets),