Statnote 33: the intra-class correlation coefficient

Anthony Hilton, Richard Armstrong

Research output: Contribution to specialist publication or newspaperArticle


The intra-class correlation coefficient (ICC or ri) is a method of measuring correlation when the data are paired and therefore, should be used when experimental units are organised into groups. A useful analogy is with the unpaired or paired ‘t’ test to compare the differences between the means of two groups. In studies of reproducibility, there may actually be little difference between the ICC and Pearson’s ‘r’ for ‘true’ repeated measurements. If, however, there is a systematic change in the measurements made on the first compared with the second occasion, then the ICC will be significantly less than ‘r’, and less confidence would be placed in the reproducibility of the results.
Original languageEnglish
Number of pages2
Specialist publicationMicrobiologist
Publication statusPublished - Jun 2013


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