Kappa statistic

In their excellent overview of a common statistical measure of agreement, Thomas McGinn and colleagues1 suggest in Table 1 that values for the kappa statistic range from 0 to 1. However, negative values of kappa are also possible.2 Although unusual in practice, a negative kappa statistic results when agreement occurs less often than predicted by chance alone. This may indicate genuine disagreement, or it may reflect a problem in the application of a diagnostic test. Readers and researchers who encounter a negative kappa statistic should be aware of its implications, rather than blaming mathematical or typographic errors or computer “gnomes.”