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The intra-class correlation is used to estimate the correlation of one variable between two members within a group, for instance between two children of one family. This is in contrast to Pearson's Correlation, where the variables of interest are modeled as two distinct traits, with the mean and variance of each being estimated separately. In the intraclass correlation, the trait's mean and variance are derived from pooled estimates across all members of all groups. Because of this, the intraclass correlation gives the proportion of variance attributable to between group differences. In the example of siblings nested in families, the intraclass correlation gives the proportion of variance accounted for by family membership, while the Pearson gives the proportion of shared variance between the two members of a pair without respect to group (family) membership. You might think of it as the equivalent of a matched-sample t-test.
Early definition Edit
where N is the degrees of freedom (Note that the precise form of the formula differ between versions of Fisher's book: The 1954 edition uses in places where the 1925 edition uses ). This form is not the same as the interclass correlation. For the data set with two groups the intraclass correlation r will be confined to the interval [-1, +1].
The intraclass correlation is also defined for data sets with more than two groups, e.g., for three groups it is computed as
(Also this form differs between editions of Fisher's book)
As the number of groups grow, the number of terms in the form will grow exponentially, but another form has been suggested that does not require so many computations
where K is the number of groups. This form is usually attributed to Harris. The left term is non-negative, consequently the intraclass correlation must be
"Modern" ICCs Edit
Beginning with Ronald Fisher the intraclass correlation has been regarded within the framework of analysis of variance (ANOVA). Different ICCs arise with different ANOVA models, e.g., one-way analysis or two-way analysis, and they may produce marked different results. An article by McGraw and Wong lists these variations.
The open-source R-Project may also be used to compute the intraclass correlation (package 'psy').
Other methods of measuring agreement/correlation Edit
- ↑ Koch, Gary G. (1982). "Intraclass correlation coefficient". Encyclopedia of Statistical Sciences 4. Ed. Samuel Kotz and Norman L. Johnson. New York: John Wiley & Sons. 213–217.
- ↑ Reinhold Müller & Petra Büttner (December 1994). A critical discussion of intraclass correlation coefficients. Statistics in Medicine 13 (23-24): 2465–2476. See also comment:
- P. Vargha (1997). Letter to the Editor. Statistics in Medicine 16: 821–823.
- ↑ 3.0 3.1 3.2 3.3 3.4 Ronald A. Fisher (1954). Statistical Methods for Research Workers, Twelfth edition, Oliver and Boyd.
- ↑ Ronald A. Fisher (1925). Statistical Methods for Research Workers, Oliver and Boyd.
- ↑ J. Arthur Harris (October 1913). On the Calculation of Intra-Class and Inter-Class Coefficinets of Correlation from Class Moments when the Number of Possible Combinations is Large 9 (3/4): 446–472.
- ↑ Kenneth O. McGraw & S. P. Wong (1996). Forming inferences about some intraclass correlation coefficients. Psychological Methods 1: 30–46. There are several errors in the article:
- ↑ Carol A. E. Nickerson (December 1997). A Note on 'A Concordance Correlation Coefficient to Evaluate Reproducibility'. Biometrics 53: 1503–1507.
- ↑ Richard N. MacLennan (November 1993). Interrater Reliability with SPSS for Windows 5.0. The American Statistician 47 (4): 292–296.
Other references Edit
- Michael T. Brannick, "Shrout and Fleiss Computations for Intraclass correlations for interjudge reliability"
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