One step further: evaluating interpreters using metamorphic testing

The black-box nature of the Deep Neural Network (DNN) makes it difficult for people to understand why it makes a specific decision, which restricts its applications in critical tasks. Recently, many interpreters (interpretation methods) are proposed to improve the transparency of DNNs by providing relevant features in the form of a saliency map. However, different interpreters might provide different interpretation results for the same classification case, which motivates us to conduct the robustness evaluation of interpreters. However, the biggest challenge of evaluating interpreters is the testing oracle problem, i.e., hard to label ground-truth interpretation results. To fill this critical gap, we first use the images with bounding boxes in the object detection system and the images inserted with backdoor triggers as our original ground-truth dataset. Then, we apply metamorphic testing to extend the dataset by three operators, including inserting an object, deleting an object, and feature squeezing the image background. Our key intuition is that after the three operations which do not modify the primary detected objects, the interpretation results should not change for good interpreters. Finally, we measure the qualities of interpretation results quantitatively with the Intersection-over-Minimum (IoMin) score and evaluate interpreters based on the statistics of metamorphic relation's failures. We evaluate seven popular interpreters on 877,324 metamorphic images in diverse scenes. The results show that our approach can quantitatively evaluate interpreters' robustness, where Grad-CAM provides the most reliable interpretation results among the seven interpreters.