Background: In this article, we describe qualitative and quantitative methods for assessing the degree of agreement (concordance) between two measuring or rating techniques. An assessment of concordance is particularly important when a new measuring technique is introduced.
Methods: We give an example to illustrate a number of simple methods of comparing different measuring or rating techniques, and we explain the underlying principle of each method. We also give further illustrative examples from medical research papers that were retrieved by a selective literature search.
Results: Methods of comparing different measuring or rating techniques are of two kinds: those with a nominal rating scale and those with a continuous rating scale. We only discuss methods for comparing one measuring or rating technique with another one. Moreover, we point out some common erroneous approaches to concordance analysis.
Conclusion: Concordance analysis is needed to establish the validity of a new diagnostic measuring or rating technique or to demonstrate the near-equivalence of multiple measuring or rating techniques. Erroneous approaches to concordance analysis can lead to false conclusions.