Abstract
In CTT, reliability is defined as the proportion of true score variance to total variance. It is most often estimated using the coefficient \( \alpha \). This index assumes the instrument is unidimensional and is not a test of unidimensionality. Construct validation addresses the substantive dimension of the variable assessed.
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Further Reading
Andrich, D. (1988). Rasch models for measurement (pp. 84–86). Newbury Park, CA: Sage.
Andrich, D. (2016). Components of variance of scales with a bi-factor structure from two calculations of coefficient alpha. Educational Measurement: Issues and Practice,35(4), 25–30.
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Exercises
Exercises
In the Exercises of Chap. 3, you were given a table of person–item responses.
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1.
Calculate the variance of each of the eight items in the test and the total score and summarize them as below:
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2.
Calculate the reliability of this test according to coefficient \( \alpha \). Show your working. Use the variances of the eight items and the variance of the total score that you calculated in question 1.
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3.
Comment on the size of the reliability.
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4.
Consider a test or examination with which you are familiar with. Describe the test and its purposes first, then comment on the reliability of the examination and the validity in terms of the various functions the examination is supposed to serve. How might these be investigated?
For further exercises, see Exercise 1: Interpretation of RUMM2030 printout in Appendix C.
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Andrich, D., Marais, I. (2019). Reliability and Validity in Classical Test Theory. In: A Course in Rasch Measurement Theory. Springer Texts in Education. Springer, Singapore. https://doi.org/10.1007/978-981-13-7496-8_4
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