Fig. 2

a First two dimensions of principal coordinates analysis (PCoA) of a simulated outlier and 29 reference microbiomes. Even though the outlier has no species in common with the reference samples, two-dimensional PCoA obscures the fact that it is an outlier, b PC6 PCoA axe plotted against PC1, showing that in this particular case six dimensions are sufficient to observe the outlier. In real clinical data, the true number of dimensions required is not known. c Example of an outlier that would not be detected by a centroid-based test