Before new tests are implemented, it is important to compare their results with those of other measurement methods that are already in use. In the determination of this so-called agreement between methods, one may choose between several statistical approaches. The correlation coefficient is a popular approach to determine the agreement between measurement methods. It is easy to calculate, but has important limitations: it does not provide any information on the type of association and it is extremely sensitive to the range of values within the study. Finally, a correlation coefficient does not reveal whether any difference between two measurements is systematic or random. Therefore, it is highly preferable to use Bland-Altman plots instead, as these reveal both systematic and random errors. Bland-Altman plots are also preferable in case of repeated measurements and calibrations.