What is reliability and validity in psychology?

In the context of psychology, reliability and validity may refer either to measurement tools such as questionnaires or to research studies. Reliability is a measure of the consistency of the results obtained from a psychological tool or study. In order for a measurement scale to be regarded as highly reliable, it should yield the same results if the conditions remain constant. For example, if the same participant is tested in the same conditions repeatedly, then consistent results should be produced.

Validity refers to the capacity of a tool to measure what it claims to measure. A questionnaire designed to test for aggression cannot measure depression, just as a weighting scale cannot measure height.