Several factors can affect the reliability of a test (Mehrens & Lehmann, 1991; Sattler, 2001):
- Test length. Generally, the longer a test is, the more reliable it is.
- Speed. When a test is a speed test, reliability can be problematic. It is inappropriate to estimate reliability using internal consistency, test-retest, or alternate form methods. This is because not every student is able to complete all of the items in a speed test. In contrast, a power test is a test in which every student is able to complete all the items.
- Group homogeneity. In general, the more heterogeneous the group of students who take the test, the more reliable the measure will be.
- Item difficulty. When there is little variability among test scores, the reliability will be low. Thus, reliability will be low if a test is so easy that every student gets most or all of the items correct or so difficult that every student gets most or all of the items wrong.
- Objectivity. Objectively scored tests, rather than subjectively scored tests, show a higher reliability.
- Test-retest interval. The shorter the time interval between two administrations of a test, the less likely that changes will occur and the higher the reliability will be.
- Variation with the testing situation. Errors in the testing situation (e.g., students misunderstanding or misreading test directions, noise level, distractions, and sickness) can cause test scores to vary.
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