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Title: A Statistical Approach to Assessing Neural Network Robustness

Paper content:
We present a new approach to assessing the robustness of neural networks based on estimating the proportion of inputs for which a property is violated.
Specifically, we estimate the probability of the event that the property is violated under an input model.
Our approach critically varies from the formal verification framework in that when the property can be violated, it provides an informative notion of how robust the network is, rather than just the conventional assertion that the network is not verifiable.
Furthermore, it provides an ability to scale to larger networks than formal verification approaches.
Though the framework still provides a formal guarantee of satisfiability whenever it successfully finds one or more violations, these advantages do come at the cost of only providing a statistical estimate of unsatisfiability whenever no violation is found.
Key to the practical success of our approach is an adaptation of multi-level splitting, a Monte Carlo approach for estimating the probability of rare events, to our statistical robustness framework.
We demonstrate that our approach is able to emulate formal verification procedures on benchmark problems, while scaling to larger networks and providing reliable additional information in the form of accurate estimates of the violation probability.
The robustness of deep neural networks must be guaranteed in mission-critical applications where their failure could have severe real-world implications.
This motivates the study of neural network verification, in which one wishes to assert whether certain inputs in a given subdomain of the network might lead to important properties being violated BID29 BID1 .
For example, in a classification task, one might want to ensure that small perturbations of the inputs do not lead to incorrect class labels being predicted BID23 BID8 .The
classic approach to such verification has focused on answering the binary question of whether there exist any counterexamples that violate the property of interest. We
argue that this approach has two major drawbacks. Firstly
, it provides no notion of how robust a network is whenever a counterexample can be found. Secondly
, it creates a computational problem whenever no counterexamples exist, as formally verifying this can be very costly and does not currently scale to the size of networks used in many applications.To give a demonstrative example, consider a neural network for classifying objects in the path of an autonomous vehicle. It will
almost certainly be infeasible to train such a network that is perfectly robust to misclassification. Furthermore
, because the network will most likely need to be of significant size to be effective, it is unlikely to be tractable to formally verify the network is perfectly robust, even if such a network exists. Despite this
, it is still critically important to assess the robustness of the network, so that manufacturers can decide whether it is safe to deploy.To address the shortfalls of the classic approach, we develop a new measure of intrinsic robustness of neural networks based on the probability that a property is violated under an input distribution model. Our measure
is based on two key insights. The first is
that for many, if not most, applications, full formal verification is neither necessary nor realistically achievable, such that one actually desires a notion of how robust a network is to a set of inputs, not just a binary answer as to whether it is robust or not. The second is
that most practical applications have some acceptable level of risk, such that it is sufficient to show that the probability of a violation is below a certain threshold, rather than confirm that this probability is exactly zero.By providing a probability of violation, our approach is able to address the needs of applications such as our autonomous vehicle example. If the network
is not perfectly robust, it provides an explicit measure of exactly how robust the network is. If the network
is perfectly robust, it is still able to tractability assert that a violation event is "probably-unsatisfiable". That is it is
able to statistically conclude that the violation probability is below some tolerance threshold to true zero, even for large networks for which formal verification would not be possible.Calculating the probability of violation is still itself a computationally challenging task, corresponding to estimating the value of an intractable integral. In particular
, in most cases, violations of the target property constitute (potentially extremely) rare events. Consequently
, the simple approach of constructing a direct Monte Carlo estimate by sampling from the input model and evaluating the property will be expensive and only viable when the event is relatively common. To address
this, we adapt an algorithm from the Monte Carlo literature, adaptive multi-level splitting (AMLS) BID9 BID18 , to our network verification setting. AMLS is explicitly
designed for prediction of rare events and our adaptation means that we are able to reliably estimate the probability of violation, even when the true value is extremely small.Our resulting framework is easy to implement, scales linearly in the cost of the forward operation of the neural network, and is agnostic both to the network architecture and input model. Assumptions such as
piecewise linearity, Lipschitz continuity, or a specific network form are not required. Furthermore, it produces
a diversity of samples which violate the property as a side-product. To summarize, our main contributions
are:• Reframing neural network verification as the estimation of the probability of a violation, thereby providing a more informative robustness metric for non-verifiable networks; • Adaptation of the AMLS method to our verification framework to allow the tractable estimation of our metric for large networks and rare events; • Validation of our approach on several models and datasets from the literature.
We have introduced a new measure for the intrinsic robustness of a neural network, and have validated its utility on several datasets from the formal verification and deep learning literatures.
Our approach was able to exactly emulate formal verification approaches for satisfiable properties and provide high confidence, accurate predictions for properties which were not.
The two key advantages it provides over previous approaches are:
a) providing an explicit and intuitive measure for how robust networks are to satisfiable properties; and
b) providing improved scaling over classical approaches for identifying unsatisfiable properties.Despite providing a more informative measure of how robust a neural network is, our approach may not be appropriate in all circumstances.
In situations where there is an explicit and effective adversary, instead of inputs being generated by chance, we may care more about how far away the single closest counterexample is to the input, rather than the general prevalence of counterexamples.
Here our method may fail to find counterexamples because they reside on a subset with probability less than P min ; the counterexamples may even reside on a subset of the input space with measure zero with respect to the input distribution.
On the other hand, there are many practical scenarios, such as those discussed in the introduction, where either it is unrealistic for there to be no counterexamples close to the input, the network (or input space) is too large to realistically permit formal verification, or where potential counterexamples are generated by chance rather than by an adversary.
We believe that for these scenarios our approach offers significant advantages to formal verification approaches.Going forward, one way the efficiency of our approach could be improved further is by using a more efficient base MCMC kernel in our AMLS estimator, that is, replace line 12 in Algorithm 1 with a more efficient base inference scheme.
The current MH scheme was chosen on the basis of simplicity and the fact it already gave effective empirical performance.
However, using more advanced inference approaches, such as gradient-based approaches like Langevin Monte Carlo (LMC) BID21 Hamiltonian Monte Carlo (Neal, 2011) , could provide significant speedups by improving the mixing of the Markov chains, thereby reducing the number of required MCMC transitions.

assistant: text: We introduce a statistical approach to assessing neural network robustness that provides an informative notion of how robust a network is, rather than just the conventional binary assertion of whether or not of property is violated.