Reliability is an essential element of test quality. An instrument for measurement is reliable if it provides consistent results. But a reliable instrument need not be valid. For example, if a clock shows time nonstop then it is reliable, but that does not mean it is showing the correct time. Reliability deals with consistency, or reproducibility of similar results in a test by the test subject, if a test is administered on two occasions; the same conclusions are reached both times. While a test with poor reliability will have remarkably different scores each time with the same test and same examinee.
If a test is then it has to be reliable, but the vice versa is not true. Although, reliability might is not as valuable as validity, but nonetheless reliability it is easier to assess than validity for a test. Reliability has two key aspects: stability and equivalence. The degree of stability can be located comparing the results of repeated measurements with the same candidate and the same instrument. Equivalence means the probability of the amount of errors getting introduced by various investigators or different sample items being studied during the repetition of the test. The best way to test for reliability of a test is that two investigators should compare their observations of the same events. Reliability can be improved in the following ways:
(i) By standardizing the measurement conditions to reduce external factors such as boredom, fatigue, etc. which leads to achievement of stability.
(ii) By detailed directions for measurement which can be generalized and used by trained and motivated persons to conduct research and also by increasing the purview of the sample of items used, this lead to equivalence.
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