Cohen’s Kappa statistic is a very useful, but under-utilised, metric. Sometimes in machine learning we are faced with a multi-class classification problem. In those cases, measures such as the accuracy, or precision/recall do not provide the complete picture of the performance of our classifier.
In some other cases we might face a problem with imbalanced classes. E.g. we have two classes, say A and B, and A shows up on 5% of the time. Accuracy can be misleading, so we go for measures such as precision and recall. There are ways to combine the two, such as the F-measure, but the F-measure does not have a very good intuitive explanation, other than it being the harmonic mean of precision and recall.
Cohen’s kappa statistic is a very good measure that can handle very well both multi-class and imbalanced class problems.
Cohen’s kappa is defined as:
where po is the observed agreement, and pe is the expected agreement. It basically tells you how much better your classifier is performing over the performance of a classifier that simply guesses at random according to the frequency of each class.
Cohen’s kappa is always less than or equal to 1. Values of 0 or less, indicate that the classifier is useless. There is no standardized way to interpret its values. Landis and Koch (1977) provide a way to characterize values. According to their scheme a value < 0 is indicating no agreement , 0–0.20 as slight, 0.21–0.40 as fair, 0.41–0.60 as moderate, 0.61–0.80 as substantial, and 0.81–1 as almost perfect agreement.
Cohen’s kappa is provided by many software packages and libraries such as caret, Weka and scikit-learn. So, next time you face a problem with imbalanced classes or a multi-class classification problem give it a go! In the meantime, if you want to read about another interesting metric, but this time in regression, make sure to check my article about the concordance correlation coefficient.
Landis, J.R.; Koch, G.G. (1977). “The measurement of observer agreement for categorical data”. Biometrics 33 (1): 159–174