  The Receiver Operating Characteristic (ROC) is a well-established
representation of the tradeoff between detection and false alarm probabilities
in binary hypothesis testing. In many practical contexts ROC's are generated by
thresholding a measured score variable -- applying score variable threshold
tests (SVT's). In many cases the resulting curve is different from the
likelihood ratio test (LRT) ROC and is therefore not Neyman-Pearson optimal.
While it is well-understood that concavity is a necessary condition for an ROC
to be Neyman-Pearson optimal, this paper establishes that it is also a
sufficient condition in the case where the ROC was generated using SVT's. It
further defines a constructive procedure by which the LRT ROC can be generated
from a non-concave SVT ROC, without requiring explicit knowledge of the
conditional PDF's of the score variable. If the conditional PDF's are known,
the procedure implicitly provides a way of redesigning the test so that it is
equivalent to an LRT.
