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Internal consistency reliability estimates how much total test scores would vary if slightly different items were used. Researchers usually want to measure constructs rather than particular items. Therefore, they need to know whether the items have a large influence on test scores and research conclusions.

This entry begins with a discussion of classical reliability theory. Next, formulas for estimating internal consistency are presented, along with a discussion of the importance of internal consistency. Last, common misinterpretations and the interaction of all types of reliability are examined.

Classical Reliability Theory

To examine reliability, classical test score theory divides observed scores on a test into two components, true score and error:

where X = observed score, T = true score, and E = error score.

If Steve's true score on a ...

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