Paper Summary - Improving Robot Controller Transparency Through Autonomous Policy Explanation

Hayes, B., & Shah, J. A. (2017, March). Improving robot controller transparency through autonomous policy explanation. In 2017 12th ACM/IEEE International Conference on Human-Robot Interaction (HRI) (pp. 303-312). IEEE.

My Objectives

To learn about an XRL technique that I could use for my project.

Paper Summary

This paper presents a method to extract explanations from a policy. It requires learning an MDP of the policy using a dataset of real or simulated transitions. The method uses a hand-authored mapping from state observations to state predicates (communicable predicate set). Given these, it can provide explanations in the form of DNF formulas for 3 different query types -

1. When do you do {action}?

   Provide a general state description for the states in which you do {action}

2. What do you do when {state}?

   Provide the action taken by the policy in states which match {state}

3. Why didn’t you do {other-action}?

   Provide a description of why {other-action} wasn’t taken in the current state. This is simply counterfactual reasoning as described further in Madumal et. al. (2019).

The DNF formulas generated are combined with a templating system to generate natural language explanation to human users. The focus in the paper is on using the explanations to aid debugging agent behaviour. The authors provide examples of generated explanations in 3 different domains and contrast it with an expert explanation.

Methodology

Learning the agent policy

In this step, a dataset consisting of $(s, a, s’)$ tuples is generated by running the agent in the environment. The dataset is used to approximate the agent policy and learn a function to predict which action the agent will take in a given state.

Communicable Predicate Sets

These are a collection of hand-authored predicates which provide relevant state information. They may or may not map directly onto the state features provided as input to the agent. They are a set of first-order logic clauses which can be either $\text{True}$ or $\text{False}$ in a given state. An example of these in the CartPole domain are “pole falling left/right”, “pole standing up”, “cart moving left/right” etc.

These predicates are used to produce a DNF formula which represents a boolean function that is true for a given set of states, and false for another. If these groups of sets have some common property, say, if they are the sets in which a particular action is frequently taken, then the resulting DNF formula becomes the explanation for why that action was taken in terms of the communicable predicates. If we further apply the templates based on the query and predicate, we can generate a natural language explanation. For e.g., if we have predicates $\text{idle}/0$ and $\text{running}/0$ indicating whether the robot is active/idle, and if the factory line is running/stationary respectively, a DNF formula $(\text{idle} \land \lnot \text{running} \lor (\lnot \text{idle}))$ would beget the explanation “The robot is idle and the factory line is not running or the robot is active”.

The DNF formula is generated from a target set of states $S$ and an excluded set of states $\overline{S}$. The problem is equivalent to finding the minimal set cover for the states, and is solved using the Quine-McClusky algorithm (although an approximate algorithm like ESPRESSO can also be used in case of large state spaces).

The resulting DNF formula can be used to generate explanations for 3 different query types. The authors provide algorithms for each, and they involve different ways to construct the sets $S$ and $\overline{S}$ to generate the descriptions.

1. When do you do {action}?

   Using the learned agent policy, the environment’s state space is enumerated (or sampled, if large) and $S$ is constructed from every state where the target action is taken, and $\overline{S}$ from the states where it isn’t.

2. What do you do when {state}?

   Given an input state description $s$ as a partial subset of the communicable predicates, we identify the states where the description holds true. For each of these states, we find the action taken using the learned policy ($\pi(s) = a$) and use it to group them ($S[a] \leftarrow S[a] \cup s$). We then return the reason why that action was taken in that group of states $S[a]$ and not in any others.

3. Why didn’t you do {other-action}?

   Counterfactual explanations are generated by finding the set of states where the query action is the one executed by the learned policy. The algorithm in the paper scans the set of states a certain distance away from the current state. It generates a reason for why the query action was taken in those states, why the actually executed action was taken in the current state, and returns the difference between the two.

Experimental Setup

The authors conduct experiments using 3 different environments with different agents. For each environment, policy explanations were generated in response to queries of all three types.

1. GridWorld (discrete, planning)

   The authors trained a tabular $\epsilon$-greedy Q-learning agent.

2. CartPole (continuous)

   The authors trained a neural network-based agent using Q-learning.

3. Robot Inspection (multi-agent, complex, dynamic)

   The authors wrote a traditional conditional-logic based agent for this environment. It is unclear whether the environment was simulated or physical.

Results and Analysis

The authors generate a policy summary for each domain and compare it to a policy summary written by a human expert in terms of the communicable predicate sets for the domain. The comparison is qualitative and presented in terms of “closeness” to the expert summary.

The authors present an example of a counterfactual explanation generated for the robot inspection domain.

Discussion

The authors use evidence of their method being able to generate explanations in a variety of environments and for a variety of agents as confirmation of their hypothesis. They present qualitative arguments for how it can improve policy understanding and aid in debugging agent behaviour.

The authors propose as future work -

1. using parsed natural language descriptions of valid task strategies to map advice down to state regions for biasing exploration during policy learning

   This is unclear, but since it deals with a better way to learn the agent policy, it is not too important to understand since any SOTA method will work.

2. explaining human behaviour using inverse RL

Impressions

Unnecessary discussion of logging

This paper devotes an entire section to describe how they add logging to an existing agent code in order to obtain the $(s, a, s’)$ tuples used to learn the agent policy. This is unnecessary since most RL setups utilize an environment which includes an interface to input the action and receive the reward and the next state. This logging is already in-built into most RL libraries in use today like OpenAI Gym. It is a solved problem.

Lack of explanation evaluation

This paper does not have a robust evaluation of the generated explanations. The focus of the paper being using the explanations to understand and debug agent behaviour, it seems strange that tasks which involve predicting agent behaviour (to test understanding) or debugging are not evaluated using human studies. I’m not sure what task the latter would involve. An idea is to provide an agent which behaves contrary to a specification and have users query it for explanations until the cause is found. Their experience can be recorded using qualitative surveys.

Additionally, it seems unlikely that explanations with large communicable predicate sets will be very explainable. Even the policy descriptions presented in the paper seemed to border on being confusing and overly verbose. Given that the environments used in the experiments seemed very basic, with small state and action spaces, it is very likely that this problem will only get bigger as the environments get more complex.

Hand-authored communicable predicate sets

The explanations rely heavily on using meaningful predicates which capture relevant information in the state. In certain planning-based domains, it is possible to have a 1:1 mapping between the state features and the predicates. In other domains, it is likely that a lot of authorial design and creativity will be needed to come up with a predicate set that is -

1. representative enough of the original state to enable generation of meaningful explanations
2. meaningful to an end user

I find it a possibility that with a poorly chosen predicate set, we might get either overly short (or empty) or extremely long explanations due to the Quine-McClusky algorithm being unable to effectively minimize the given expression.

Additionally, for domains which use raw features like pixel data or audio, it is cumbersome to design features which could effectively represent such states. If used as is, the explanations would be meaningless to any human (e.g., I moved up because pixel (1, 1) and pixel (2, 1) and not pixel (3, 2) etc.)

Focus on natural language explanation

I found the focus on providing natural language queries and explanation unnecessary. Neither does the paper need it, nor does it actually use it in any meaningful way. Template-based string generation is a very crude way of doing it and not worth focusing on. The queries are also seemingly provided in terms of just the parameters since there is no mention of a parsing and identification algorithm from natural language.

Additionally, explanations need not be in purely natural language. They can comprise of any multimedia. They are akin to the presentation logic for a narrative in narrative generation. The model for an explanation (the DNF formula) should be able to generate explanations in any desired format.

A list of other minor issues I found in the paper are -

1. The environment model is stated as required to be learned but is seemingly not used anywhere else. It is possible I have misunderstood something somewhere.
2. The behaviour model $G = (V, E)$ input to the explanation generation algorithms is not adequately described beforehand.