Review suitable distance metrics

[MaxGhenis]

Analyses thus far have used Euclidean distance, which has worked well enough for initial eyeballing. However, it doesn't distinguish much between a small value and zero, which is important given the PUF's sparsity. One rule of thumb proposed is that Euclidean isn't useful when less than 3/4 of attributes are non-zero, which is certainly the case in the PUF.

That same thread suggested that cosine similarity can be better in these cases, though a comment here suggests it's best for categorical data. Cosine similarity should be normalized. Others like Gower and Mahalanobis distances can be investigated here.