Evaluating agreement between two methods for measuring the same quantity: a response.

A method comparison study is typically concerned with evaluating the agreement between a "new" method against an "established" method for measuring the same quantity. Various statistical indices have been advocated for measuring the extent to which two methods give identical readings, that is, absolute method agreement. This article scrutinizes two contending statistical indices, the intraclass correlation and the "limits of agreement", for measuring absolute method agreement, and found neither index to be without deficiency. Notwithstanding, each index can serve a useful role in quantitating method agreement in carefully considered situations.