What does it mean when a test is said to have high reliability?

A test that has high reliability indicates that it consistently measures what it is intended to measure over time. This consistency means that if the same individual takes the test multiple times, the results will be similar each time, assuming that there has been no change in their abilities or the underlying construct being assessed. Reliability is crucial as it ensures that the test yields stable and dependable results, which contributes to the test's credibility in psychological assessments and research.

The emphasis on consistency in measurements underscores the importance of reliability in evaluating tests. For example, a reliable intelligence test would yield similar scores for a person who takes the test on different occasions, provided that their underlying cognitive abilities have not changed. This characteristic is essential because it minimizes the variance in test results that could arise from random errors or fluctuations, leading to a more accurate understanding of the individual's abilities.

In contrast, tests that produce different results each time indicate low reliability, making them less trustworthy and valid for drawing conclusions about an individual's capabilities or traits. Therefore, high reliability is a foundational criterion for assessing the quality and usefulness of psychological tests.