Agreement studies in obstetrics and gynaecology: inappropriateness, controversies and consequences.

A literature review was performed to assess how agreement has been measured in obstetrics and gynaecology. Inappropriate or controversial measures were often used. The possible consequences of these inadequacies on validity studies and randomised controlled trials are shown in case examples. The association of two measures, proportions of agreement and kappa for categorical variables, and limits of agreement and intraclass correlation coefficient for continuous variables, is encouraged in agreement studies, until new and better ways of assessing agreement are found.