def recall(ntp, nfn):
if (ntp+nfn) > <NUM_LIT:0>:<EOL><INDENT>return ntp/(ntp+nfn)<EOL><DEDENT>else:<EOL><INDENT>return np.nan<EOL><DEDENT>
This calculates recall.

https://en.wikipedia.org/wiki/Precision_and_recall

Parameters
----------

ntp : int
    The number of true positives.

nfn : int
    The number of false negatives.

Returns
-------

float
    The precision calculated using `ntp/(ntp + nfn)`.