Another approach to correspondence (useful if there are only two reviewers and the scale is continuous) is to calculate the differences between each pair of observations of the two reviewers. The mean value of these differences is called “bias” and the reference interval (mean value ± 1.96 × standard deviation) is called the conformity limit. The limitations of the agreement make it possible to determine the extent to which random variations can influence evaluations. Quantifying compliance in another way inevitably involves a model of how assessments are issued and why assessors agree or disagree. This model is either explicit, as with latent structural models, or implicitly, as in the case of Kappa coefficients. .