What Is It?
Test reliability is the definition of how consistent a measure is of a particular element over a period of time, and between different participants. For example, a test measuring personality traits should yield the same answers for a subject after several times completing the test, and with a short period of time between (so long as the individual has not inherently changed personality traits).
Reliability has sub-types that must be satisfied before a test or assessment is deemed as so.
Parallel-Forms Reliability – This is measured when there are two different tests using the same content but with different equipment or procedures; if the results gained from the assessments are still the same, then parallel-forms reliability has been satisfied.
Internal Consistency Reliability – This looks at the items within a test, to assess the internal reliability between items. For example, a personality test may seem to have two or more questions that are asking the same thing. If the participant answers these similarly, then internal consistency reliability is assumed correct.
Inter-Rater Reliability – This uses two individuals to mark or rate the scores of a psychometric test, if their scores or ratings are comparable then inter-rater reliability is confirmed.
Test-Retest Reliability – This is the final sub-type and is achieved by giving the same test out at two different times and gaining the same results each time.
Other Factors
There are always going to be minor discrepancies in the overall reliability of a test, as it is near impossible to find every defect; also, individuals taking the test are able to answer freely and may have different thoughts or feelings from one day to the next. We can look at this in two ways: factors that contribute to consistency, and factors that contribute to inconsistency. Consistency is attributed to stable traits or characteristics within the individual taking the test, for example height and weight. Inconsistency is attributed to many different things, for example, the health of the participant on test day, their understanding of the test, or luck in choosing a randomly correct answer.
Why Is Reliability Important?
The reliability of a test is important, specifically when dealing with psychometric tests; there is no point in having a test that will yield different answers each time measured, particularly when it can influence the decisions of employers and who they may employ to lead their company.