Reliability refers to consistency of results over time. Reliability is one of the standards like validity, against which the tools used to measure concepts are judged. William M.K. Trochim defined reliability in statistics and psychometrics as the overall consistency of a measure. A measure is said to have a high reliability if it produces similar results under consistent conditions.
According to National Council on Measurement in Education, "It is the characteristic of a set of test scores that relates to the amount of random error from the measurement process that might be embedded in the scores. Scores that are highly reliable are accurate, reproducible, and consistent from one testing occasion to another. That is, if the testing process were repeated with a group of test takers, essentially the same results would be obtained. Various kinds of reliability coefficients, with values ranging between 0.00 (much error) and 1.00 (no error), are usually used to indicate the amount of error in the scores."
Are I.Q. tests a reliable measure of intelligence?
Are official suicide statistics reliable measures of the suicide rate?
If a bathroom scale is used to measure the concept of weight, one must ask: Is this tool, the bathroom scale, reliable? Does it provide consistent results? To check this get back on the scale a second time to see if it produces the same results. We must note that the bathroom scale may be reliable and yet be inaccurate.
Are questions about which political party a person would vote for a reliable measure of political preference? Since in many of these examples it is difficult to assume, like weight, that the results would remain the same over time, it may be more correct to think of reliability as indicating consistency of results among users of the tool or measurement.
Reliability is also the ability of a person or system to perform and maintain its functions in routine circumstances, as well as hostile or unexpected circumstances.