This refers to how consistent a study or measuring device is. A measurement is said to be reliable or consistent if the measurement can produce similar results if used again in similar circumstances.
There are two types of reliability – internal and external reliability.
Internal reliability refers to the extent to which a measure is consistent within itself. The internal reliability of self-report measures, such as psychometric tests and questionnaires can be assessed using the split half method. This involves splitting a test into two and having the same participant doing both halves of the test. If the two halves of the test provide similar results this would suggest that the test has internal reliability.
External reliability refers to the extent to which a measure varies from one use to another. The external reliability of self-report measures, such as psychometric tests and questionnaires can be assessed using the test-retest method. This involves testing the same participant twice over a period of time on the same test. Similar scores would suggest that the test has external reliability.
A common way of assessing the external reliability of observations is to use inter-rater reliability. This involves comparing the ratings of two or more observers and checking for agreement in their measurements. Inter-rater reliability can also be used for interviews.