  A generalization of Passing-Bablok regression is proposed for comparing
multiple measurement methods simultaneously. Possible applications include
assay migration studies or interlaboratory trials. When comparing only two
methods, the method reduces to the usual Passing-Bablok estimator. It is close
in spirit to reduced major axis regression, which is, however, not robust. To
obtain a robust estimator, the major axis is replaced by the (hyper-)spherical
median axis. The method is shown to reduce to the usual Passing-Bablok
estimator if only two methods are compared. This technique has been applied to
compare SARS-CoV-2 serological tests, bilirubin in neonates, and an in vitro
diagnostic test using different instruments, sample preparations, and reagent
lots. In addition, plots similar to the well-known Bland-Altman plots have been
developed to represent the variance structure.
