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. 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 I.Q. tests a reliable measure of intelligence?
Are official suicide statistics reliable measures of the suicide rate?
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.