  Covariate shift may impact the operational safety performance of neural
networks. A re-evaluation of the safety performance, however, requires
collecting new operational data and creating corresponding ground truth labels,
which often is not possible during operation. We are therefore proposing to
reshape the initial test set, as used for the safety performance evaluation
prior to deployment, based on an approximation of the operational data. This
approximation is obtained by observing and learning the distribution of
activation patterns of neurons in the network during operation. The reshaped
test set reflects the distribution of neuron activation values as observed
during operation, and may therefore be used for re-evaluating safety
performance in the presence of covariate shift. First, we derive conservative
bounds on the values of neurons by applying finite binning and static dataflow
analysis. Second, we formulate a mixed integer linear programming (MILP)
constraint for constructing the minimum set of data points to be removed in the
test set, such that the difference between the discretized test and operational
distributions is bounded. We discuss potential benefits and limitations of this
constraint-based approach based on our initial experience with an implemented
research prototype.
