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Intraclass correlation coefficient


The Intraclass Correlation Coefficient (ICC) is a measure of the reliability of measurements or ratings.

For the purpose of assessing inter-rater reliability and the ICC, two or preferably more raters rate a number of study subjects.

  • A distinction is made between two study models: (1) each subject is rated by a different and random selection of a pool of raters, and (2) each subject is rated by the same raters.
  • In the first model, the ICC is always a measure for Absolute agreement; in the second model a choice can be made between two types: Consistency when systematic differences between raters are irrelevant, and Absolute agreement, when systematic differences are relevant.
    For example: the paired ratings (2,4), (4,6) and (6,8) are in perfect agreement, with a consistency of 1.0, but with an absolute agreement of 0.6667.

How to enter data

Intraclass correlation coefficient - How to enter data

In this example (taken from Shrout PE & Fleiss JL, 1979) data are available for 4 raters on 6 subjects. The data for each subject are entered in the 4 columns.

If not all subjects are rated by the same 4 raters, the data are still entered in 4 columns, the order of which then being unimportant.

Required input

Intraclass correlation coefficient - dialog box

  • Measurements: variables that contain the measurements of the different raters.
  • Filter: an optional filter to include only a selected subgroup of cases.
  • Options
    • Model
      • Raters for each subject were selected at random: the raters were not the same for all subjects, a random selection or raters rated each subject.
      • The same raters for all subjects: all subjects were rated by the same raters.
    • Type
      • Consistency: systematic differences between raters are irrelevant.
      • Absolute agreement: systematic differences are relevant


Intraclass correlation coefficient - results

The Intraclass correlation coefficient table reports two coefficients with their respective 95% Confidence Interval.

  • Single measures: this ICC is an index for the reliability of the ratings for one, typical, single rater.
  • Average measures: this ICC is an index for the reliability of different raters averaged together. This ICC is always higher than the Single measures ICC.


  • McGraw KO, Wong SP (1996) Forming inferences about some intraclass correlation coefficients. Psychological Methods 1:30-46. (Correction: 1:390).
  • Shrout PE, Fleiss JL (1979) Intraclass correlations: uses in assessing rater reliability. Psychological Bulletin 86:420-428. PubMed

See also

External links