BackgroundThere are limitations of inter-class correlation (usually, the Pearson’s product-moment correlation) in evaluating reliability of an instrument. An intra-class correlation approach would be appropriate to examine the test-retest reliability of an instrument.ObjectivesTo introduce an intra-class correlation approach to examine the test-retest reliability of an instrument.MethodThe method of constructing an intra-class correlation coefficient was presented.ResultsThe instrument with visual analog scales to measure the perceived competence to practice breast self-examination and perceived barriers to breast self-examination was tested. The intra-class correlation coefficient was 0.640.ConclusionThe intra-class correlation is an alternative to test the reliability of an instrument and it is more sensitive to the detection of systematic error.