Homogeneity score test of AC1 statistics and estimation of common AC1 in multiple or stratified inter-rater agreement studies.

BACKGROUND: Cohen's kappa coefficient is often used as an index to measure the agreement of inter-rater determinations. However, kappa varies greatly depending on the marginal distribution of the target population and overestimates the probability of agreement occurring by chance. To overcome these limitations, an alternative and more stable agreement coefficient was proposed, referred to as Gwet's AC1. When it is desired to combine results from multiple agreement studies, such as in a meta-analysis, or to perform stratified analysis with subject covariates that affect agreement, it is of interest to compare several agreement coefficients and present a common agreement index. A homogeneity test of kappa was developed; however, there are no reports on homogeneity tests for AC1 or on an estimator of common AC1. In this article, a homogeneity score test for AC1 is therefore derived, in the case of two raters with binary outcomes from K independent strata and its performance is investigated. An estimation of the common AC1 between strata and its confidence intervals is also discussed. METHODS: Two homogeneity tests are provided: a score test and a goodness-of-fit test. In this study, the confidence intervals are derived by asymptotic, Fisher's Z transformation and profile variance methods. Monte Carlo simulation studies were conducted to examine the validity of the proposed methods. An example using clinical data is also provided. RESULTS: Type I error rates of the proposed score test were close to the nominal level when conducting simulations with small and moderate sample sizes. The confidence intervals based on Fisher's Z transformation and the profile variance method provided coverage levels close to nominal over a wide range of parameter combination. CONCLUSIONS: The method proposed in this study is considered to be useful for summarizing evaluations of consistency performed in multiple or stratified inter-rater agreement studies, for meta-analysis of reports from multiple groups and for stratified analysis.