  Presentation attack (spoofing) detection (PAD) typically operates alongside
biometric verification to improve reliablity in the face of spoofing attacks.
Even though the two sub-systems operate in tandem to solve the single task of
reliable biometric verification, they address different detection tasks and are
hence typically evaluated separately. Evidence shows that this approach is
suboptimal. We introduce a new metric for the joint evaluation of PAD solutions
operating in situ with biometric verification. In contrast to the tandem
detection cost function proposed recently, the new tandem equal error rate
(t-EER) is parameter free. The combination of two classifiers nonetheless leads
to a \emph{set} of operating points at which false alarm and miss rates are
equal and also dependent upon the prevalence of attacks. We therefore introduce
the \emph{concurrent} t-EER, a unique operating point which is invariable to
the prevalence of attacks. Using both modality (and even application) agnostic
simulated scores, as well as real scores for a voice biometrics application, we
demonstrate application of the t-EER to a wide range of biometric system
evaluations under attack. The proposed approach is a strong candidate metric
for the tandem evaluation of PAD systems and biometric comparators.
