A major part of any quality program is the assurance that the products are in accordance with the performance objectives. Reliability can be estimated through a number of methods that are grouped into two types: single-administration and multiple-administration.
Multiple-administration methods require two assessments. In the test-retest method, reliability is estimated as the correlation of the Pearson product-moment coefficient between two administrations of the same measure. In the alternate forms method, reliability is estimated by the correlation of the Pearson product-moment coefficient between two different forms of a measure administrated together. The idea behind test/retest is that one should get the same score on test 1 as well as in the test 2.
The three main components to this method are as follows:
1. Apply the measurement instrument at two separate times for each subject.
2. Calculate the correlation between the two separate measurements.
3. Assume there is no change in the underlying condition between test 1 and 2.
Single-administration methods include the split-half and internal consistency reliability methods. The split-half method considers the two halves of a measure as alternate forms. This "halves reliability" estimate is then moved to the full test length using the Spearman-Brown prediction formula. Internal consistency estimate groups questions in a questionnaire to estimate reliability. For example, one could write two sets of four questions that measure the same concept (example, class performance) and after collecting the responses, run a correlation between those two groups of four questions to decide if the instrument is consistently measuring the concept. The most common internal consistency estimate is Cronbach’s alpha, which is usually interpreted as the mean of all possible split-half coefficients.Clarity of expression (for written assessments), increasing the measure and informal