Matthews Correlation Coefficient (MCC) metric for OTU clustering

See also
Comments on Westcott & Schloss 2017
Does MCC consider unique sequence abundance?

Westcott and Schloss define the Matthews Correlation Coefficient (MCC) for OTUs as follows.

The variables are (pairs = pairs of sequences from the input data):

TP = number of pairs in the same cluster which have >=97% identity
TN = number of pairs in different clusters which have <97% identity
FP = number of pairs in the same cluster which have >97% identity
FN = number of pairs in different clusters which have >=97% identity

In general, it is not possible to construct error-free OTUs as defined by MCC, and in some simple cases MCC is undefined and fails to identify the best clusters .