Science strives to achieve safe knowledge, i.e. insights we can trust. To the extent that a piece of knowledge depends on a method, we therefore require that the method be precise, i.e. it reliably leads to a certain result. A scientific method is reliable to the extent that it produces the same results on repeated application. Reliability is, thus, a measure of the precision of a method and of the stability of its results.

In an empirical investigation, a researcher R applies a method M – possibly involving a (measuring) instrument I – to a set of data D. All of these components must be reliable:

The reliability of a method may be ascertained as a value between 0 and 1. The value required depends on the goals of the investigation.

A method can be valid only if it is reliable; but it may be reliable without being valid. In other words, reliability is necessary, but not sufficient for validity.