[II: Difference plots or scattergram--when to use which?].

Sometimes the statistical comparison of diagnostic measurement methods or subsequent measurement series in the same individuals is based on bivariate scattergrams; correlation coefficients and linear regression lines are provided in addition. However, this strategy for data representation must be considered inappropriate, as soon as the measurement series under consideration show the same physical index (e.g. repeated measurement of the intraocular pressure [mm Hg] 2 and 4 weeks after glaucoma surgery). Such data can be handled by computation of the intraindividual difference of an individual's measurement. The resulting series of intraindividual deviations can then be analysed using standard strategies: description can be based on the intraindividual differences' median and quartiles, graphical representation on the differences' distribution's box whisker plot. This procedure is particularly attractive for data arising from ophthalmological studies, when two eyes of one patient are to be compared. The statistical comparison of diagnostic methods can further be based on the Bland/Altman diagram, which not only illustrates the distribution of differences, but also motivates a dependence between the order of deviations and the absolute order of the measurement values. Whereas a box plot of intraindividual differences and a Bland/Altman diagram illustrate the concordance of two measurement series, the correlation scattergram rather visualizes their reproducibility.