The assessment of a trait or feature against a standard scale.

Psychologists rely heavily on measurements for very different purposes, ranging from clinical diagnoses based on test scores to the effects of an independent variable on a dependent variable in an experiment. Several different issues arise when considering measurement. One consideration is whether the measurement shows reliability and validity. Reliability refers to consistency: if the results of a test or measurement are reliable, a person should receive a similar score if tested on different occasions. Validity refers to whether the measurement will be useful for the purposes for which it is intended.

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