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Parallel-forms method: The key to this method is the development of alternate test forms that are equivalent in terms of content, response processes and statistical characteristics. For example, alternate forms exist for several tests of general intelligence, and these tests are generally seen equivalent. [7]
In the case when scores are not tau-equivalent (for example when there is not homogeneous but rather examination items of increasing difficulty) then the KR-20 is an indication of the lower bound of internal consistency (reliability). The formula for KR-20 for a test with K test items numbered i = 1 to K is
Predicted reliability, ′, is estimated as: ′ = ′ + ′ where n is the number of "tests" combined (see below) and ′ is the reliability of the current "test". The formula predicts the reliability of a new test composed by replicating the current test n times (or, equivalently, creating a test with n parallel forms of the current exam).
A quantity similar (but not mathematically equivalent) to congeneric reliability first appears in the appendix to McDonald's 1970 paper on factor analysis, labeled . [2] In McDonald's work, the new quantity is primarily a mathematical convenience: a well-behaved intermediate that separates two values.
The test is appropriate for individuals aged 5 years through adult. The WRAT5 provides two equivalent forms (Blue and Green), which enables retesting within short periods of time without potential practice effects that occur from repeating the same items. The alternate forms also may be administered together in a single examination. [1]
Perhaps the most commonly used index of reliability is Cronbach's α, which is equivalent to the mean of all possible split-half coefficients. Other approaches include the intra-class correlation, which is the ratio of variance of measurements of a given target to the variance of all targets. There are a number of different forms of validity.
Cronbach's alpha (Cronbach's ), also known as tau-equivalent reliability or coefficient alpha (coefficient ), is a reliability coefficient and a measure of the internal consistency of tests and measures. [1] [2] [3] It was named after the American psychologist Lee Cronbach.
Traditionally, the reliability of an assessment is based on the following: Temporal stability: Performance on a test is comparable on two or more separate occasions. Form equivalence: Performance among examinees is equivalent on different forms of a test based on the same content.