  Deep neural networks (DNNs) have shown to perform very well on large scale
object recognition problems and lead to widespread use for real-world
applications, including situations where DNN are implemented as "black boxes".
A promising approach to secure their use is to accept decisions that are likely
to be correct while discarding the others. In this work, we propose DOCTOR, a
simple method that aims to identify whether the prediction of a DNN classifier
should (or should not) be trusted so that, consequently, it would be possible
to accept it or to reject it. Two scenarios are investigated: Totally Black Box
(TBB) where only the soft-predictions are available and Partially Black Box
(PBB) where gradient-propagation to perform input pre-processing is allowed.
Empirically, we show that DOCTOR outperforms all state-of-the-art methods on
various well-known images and sentiment analysis datasets. In particular, we
observe a reduction of up to $4\%$ of the false rejection rate (FRR) in the PBB
scenario. DOCTOR can be applied to any pre-trained model, it does not require
prior information about the underlying dataset and is as simple as the simplest
available methods in the literature.
