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Test-retest reliability or Repeatability  is the variation in measurements taken by a single person or instrument on the same item and under the same conditions. A measurement may be said to be repeatable when this variation is smaller than some agreed limit. According to the Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results, repeatability conditions include:
- the same measurement procedure
- the same observer
- the same measuring instrument, used under the same conditions
- the same location
- repetition over a short period of time.
It is a statistical method used to examine how reliable a test is: A test is performed twice, e.g., the same test is given to a group of subjects at two different times. Each subject should score different than the other subjects, but if the test is reliable then each subject should score the same in both test.
Repeatability methods were developed by Bland and Altman (1986). The repeatability coefficient is a precision measure which represents the value below which the absolute difference between two repeated test results may be expected to lie with a probability of 95%. The standard deviation under repeatability conditions is part of precision and accuracy.
Test-retest reliability, is estimated as the Pearson product-moment correlation coefficient between two administrations of the same measure. This is sometimes known as the coefficient of stability
- Alternate forms reliability
- Correlation coefficient
- Split-half reliability
- Test reliability
- Types of Reliability The Research Methods Knowledge Base. Last Revised: 10/20/2006