Single-administration methods include split-half and internal consistency. In the alternate forms method, reliability is estimated by the Pearson product-moment correlation coefficient of two different forms of a measure, usually administered together. In the test-retest method, reliability is estimated as the Pearson product-moment correlation coefficient between two administrations of the same measure. Multiple-administration methods require that two assessments are administered. Reliability may be estimated through a variety of methods that fall into two types: single-administration and multiple-administration. If the scale varied a lot around 200 (190, 205, 192, 209, etc.), then the scale could be considered valid but not reliable. If the scale consistently reads '150', then it is not valid, but it is still reliable because the measurement is very consistent.

steps on the scale 10 times, and it reads '200' each time, then the measurement is reliable and valid. Īn often-used example used to elucidate the difference between reliability and validity in the experimental sciences is a common bathroom scale. It can also be interpreted as the lack of random error in measurement. It is unreliable if repeated measurements give different results. An experiment is reliable if it yields consistent results of the same measure. In experimental science, reliability is the extent to which the measurements of a test remain consistent over repeated tests of the same subject under identical conditions. In terms of accuracy and precision, reliability is precision, while validity is accuracy. For example, while there are many reliable tests of specific abilities, not all of them would be valid for predicting, say, job performance. That is, a reliable measure is measuring something consistently, but not necessarily what it is supposed to be measuring. Reliability is inversely related to random error. This can either be whether the measurements of the same instrument give or are likely to give the same measurement (test-retest), or in the case of more subjective instruments, such as personality or trait inventories, whether two independent assessors give similar scores ( inter-rater reliability).

In statistics, reliability is the consistency of a set of measurements or measuring instrument, often used to describe a test.

Psychology reliability