A literature review of the variance in 'interval length' between administrations for assessment of test-retest reliability and equivalence of PRO measures

There appears to be limited agreement about the appropriate length of interval required between two administrations of a PRO in a test-retest or equivalence study. The objective of this literature review was to determine what administration intervals are used in the development and validation of PROs, as a first step towards ascertaining the most appropriate interval between administrations for test-retest and equivalence studies for different measures. A literature review was conducted using the following search terms: test retest reliability, equivalence testing, washout period, and interval on Psychinfo since January 2003. 554 abstracts were identified. Thirty-two additional abstracts from an equivalence study review paper were included resulting in a total of 586 abstracts. All abstracts were reviewed and where available the administration interval was extracted and collated. One percent of the test retest studies had an interval of one hour or less, 18% had an interval of 1 day to 1 week, 25% had an interval of 1weeks to 2 weeks, 21% had an interval of 2 weeks to 1 month, 9% had an interval of 1 to 2 month, 13 % had an interval of 2 months or over, 13% reported a varied interval. In contrast, 50% of the equivalence studies had an administration interval of 1 hour or less, 9% had an interval of 1 hour to 1 day, 28% had an interval of 1 day to 1 week, 3% an interval of 1 to 2 weeks 10% reported a varied interval within the same study. There is a wide variety of administration intervals used in test-retest and equivalence studies seen within the literature. Considerations around the appropriate administration interval to use should be based on, amongst other things, an assessment of the stability of the condition involved and the complexity of the PRO.