Explainability in Deep Reinforcement Learning, a Review into Current Methods and Applications

The use of Deep Reinforcement Learning (DRL) schemes has increased dramatically since their first introduction in 2015. Though uses in many different applications are being found they still have a problem with the lack of interpretability. This has bread a lack of understanding and trust in the use of DRL solutions from researchers and the general public. To solve this problem the field of explainable artificial intelligence (XAI) has emerged. This is a variety of different methods that look to open the DRL black boxes, they range from the use of interpretable symbolic decision trees to numerical methods like Shapley Values. This review looks at which methods are being used and what applications they are being used. This is done to identify which models are the best suited to each application or if a method is being underutilised.