# Cohen's kappa

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Cohen's kappa coefficient is a statistical measure of inter-rater agreement for qualitative (categorical) items. It is generally thought to be a more robust measure than simple percent agreement calculation since κ takes into account the agreement occurring by chance. Some researchers (e.g. Strijbos, Martens, Prins, & Jochems, 2006) have expressed concern over κ's tendency to take the observed categories' frequencies as givens, which can have the effect of underestimating agreement for a category that is also commonly used; for this reason, κ is considered an overly conservative measure of agreement.

Others (e.g., Uebersax, 1987) contest the assertion that kappa "takes into account" chance agreement. To do this effectively would require an explicit model of how chance affects rater decisions. The so-called chance adjustment of kappa statistics supposes that, when not completely certain, raters simply guess—a very unrealistic scenario.

Nevertheless, and despite potentially better alternatives[1], Cohen's kappa enjoys continued popularity. A possible reason for this is that kappa is, under certain conditions, equivalent to the intraclass correlation coefficient.

## CalculationEdit

Cohen's kappa measures the agreement between two raters who each classify N items into C mutually exclusive categories. The first mention of a kappa-like statistic is attributed to Galton (1892), see Smeeton (1985).

The equation for κ is:

$\kappa = \frac{\Pr(a) - \Pr(e)}{1 - \Pr(e)}, \!$

where Pr(a) is the relative observed agreement among raters, and Pr(e) is the hypothetical probability of chance agreement, using the observed data to calculate the probabilities of each observer randomly saying each category. If the raters are in complete agreement then κ = 1. If there is no agreement among the raters (other than what would be expected by chance) then κ ≤ 0.

The seminal paper introducing kappa as a new technique was published by Jacob Cohen in the journal Educational and Psychological Measurement in 1960.

A similar statistic, called pi, was proposed by Scott (1955). Cohen's kappa and Scott's pi differ in terms of how Pr(e) is calculated.

Note that Cohen's kappa measures agreement between two raters only. For a similar measure of agreement (Fleiss' kappa) used when there are more than two raters, see Fleiss (1971). The Fleiss kappa, however, is a multi-rater generalization of Scott's pi statistic, not Cohen's kappa.

## ExampleEdit

Suppose that you were analyzing data related to people applying for a grant. Each grant proposal was read by two people and each reader either said "Yes" or "No" to the proposal. Suppose the data were as follows, where rows are reader A and columns are reader B:

 Yes No Yes 20 5 No 10 15

Note that there were 20 proposals that were granted by both reader A and reader B, and 15 proposals that were rejected by both readers. Thus, the observed percentage agreement is Pr(a)=(20+15)/50 = 0.70.

To calculate Pr(e) (the probability of random agreement) we note that:

• Reader A said "Yes" to 25 applicants and "No" to 25 applicants. Thus reader A said "Yes" 50% of the time.
• Reader B said "Yes" to 30 applicants and "No" to 20 applicants. Thus reader B said "Yes" 60% of the time.

Therefore the probability that both of them would say "Yes" randomly is 0.50*0.60=0.30 and the probability that both of them would say "No" is 0.50*0.40=0.20. Thus the overall probability of random agreement is Pr("e") = 0.3+0.2 = 0.5.

So now applying our formula for Cohen's Kappa we get:

$\kappa = \frac{\Pr(a) - \Pr(e)}{1 - \Pr(e)} = \frac{0.70-0.50}{1-0.50} =0.40 \!$

## Inconsistent resultsEdit

One of the problems with Cohen's Kappa is that it does not always produce the expected answer[1]. For instance, in the following two cases there is much greater agreement between A and B in the first caseTemplate:Why than in the second case and we would expect the relative values of Cohen's Kappa to reflect this. However, calculating Cohen's Kappa for each:

 Yes No Yes 45 15 No 25 15

$\kappa = \frac{0.60-0.54}{1-0.54} = 0.1304$

 Yes No Yes 25 35 No 5 35

$\kappa = \frac{0.60-0.46}{1-0.46} = 0.2593$

we find that it shows greater similarity between A and B in the second case, compared to the first.

## SignificanceEdit

Landis and Koch[1] gave the following table for interpreting $\kappa$ values. This table is however by no means universally accepted; Landis and Koch supplied no evidence to support it, basing it instead on personal opinion. It has been noted that these guidelines may be more harmful than helpful[2], as the number of categories and subjects will affect the magnitude of the value. The kappa will be higher when there are fewer categories.[3]

$\kappa$ Interpretation
< 0 No agreement
0.0 — 0.20 Slight agreement
0.21 — 0.40 Fair agreement
0.41 — 0.60 Moderate agreement
0.61 — 0.80 Substantial agreement
0.81 — 1.00 Almost perfect agreement

Threshold for reliability testing with Kappa is 0.7. K<0.7 is deemed as weak.

## NotesEdit

1. ^  Landis, J. R. and Koch, G. G. (1977) pp. 159—174
2. ^  Gwet, K. (2001)
3. ^  Sim, J. and Wright, C. C. (2005) pp. 257—268

## ReferencesEdit

1. 1.0 1.1 Kilem Gwet (May 2002). Inter-Rater Reliability: Dependency on Trait Prevalence and Marginal Homogeneity. Statistical Methods For Inter-Rater Reliability Assessment 2: ???. http://www.stataxis.com/files/articles/inter_rater_reliability_dependency.pdf
• Banerjee, M. et al. (1999). "Beyond Kappa: A Review of Interrater Agreement Measures" The Canadian Journal of Statistics / La Revue Canadienne de Statistique, Vol. 27, No. 1, pp. 3-23 <http://www.jstor.org/stable/3315487>
• Brennan, R. L. and Prediger, D. J. (1981) "Coefficient λ: Some Uses, Misuses, and Alternatives" Educational and Psychological Measurement, 41, 687-699.
• Cohen, Jacob (1960), A coefficient of agreement for nominal scales, Educational and Psychological Measurement Vol.20, No.1, pp. 37–46.
• Fleiss, J.L. (1971) "Measuring nominal scale agreement among many raters." Psychological Bulletin, Vol. 76, No. 5 pp. 378—382
• Fleiss, J. L. (1981) Statistical methods for rates and proportions. 2nd ed. (New York: John Wiley) pp. 38—46
• Fleiss, J.L. and Cohen, J. (1973) "The equivalence of weighted kappa and the intraclass correlation coefficient as measures of reliability" in Educational and Psychological Measurement, Vol. 33 pp. 613—619
• Galton, F. (1892). Finger Prints Macmillan, London.
• Gwet, K. (2001) Statistical Tables for Inter-Rater Agreement. Gaithersburg : StatAxis Publishing)
• Landis, J.R. and Koch, G. G. (1977) "The measurement of observer agreement for categorical data" in Biometrics. Vol. 33, pp. 159—174
• Scott, W. (1955). "Reliability of content analysis: The case of nominal scale coding." Public Opinion Quarterly, 17, 321-325.
• Sim, J. and Wright, C. C. (2005) "The Kappa Statistic in Reliability Studies: Use, Interpretation, and Sample Size Requirements" in Physical Therapy. Vol. 85, pp. 257—268
• Smeeton, N.C. (1985) "Early History of the Kappa Statistic" in Biometrics. Vol. 41, p.795.
• Strijbos, J., Martens, R., Prins, F., & Jochems, W. (2006). Content analysis: What are they talking about? Computers & Education, 46, 29-48.
• Uebersax JS. Diversity of decision-making models and the measurement of interrater agreement. Psychological Bulletin, 1987, 101, 140-146.