Measures such as patient reported outcome measures (PROM), used within clinical trials or service evaluation, should be reliable, valid and sensitive to detect change. These are key psychometric requirements of such tools. There are two types of reliability. The first, internal consistency, refers to how people respond to individual items designed to measure the same underlying construct. Patient reported outcome measures often consist of multiple items which are designed to measure the same underlying construct. As such, people’s responses to these items should be consistent. When this is the case, the measure can be said to have internal consistency. This is tested through Cronbach’s alpha. Cronbach’s alpha is a coefficient of reliability or consistency and is a function of the number of test items and the average inter-correlation among the items. The second form of reliability is test-retest reliability. If a measure is administered twice, a short space of time apart, it would be expected that a person’s score remains the same. The strength of correlation between the two scores is how we assess test-retest reliability. Measures need to be reliable, producing consistent scores.


How to cite: Reliability [online]. (2016). York; York Health Economics Consortium; 2016.


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