What is an epsilon-greedy policy?

An epsilon-greedy policy is a decision-making strategy used in reinforcement learning to balance exploration and exploitation. The core idea is to choose the best-known action most of the time (exploitation) while occasionally selecting a random action (exploration) to discover potentially better options. The parameter epsilon (ε), typically between 0 and 1, controls this balance. For example, if ε is 0.1, there’s a 10% chance the agent will explore randomly and a 90% chance it will take the action currently believed to yield the highest reward. This approach ensures the agent doesn’t get stuck in suboptimal behaviors by over-relying on early knowledge.

In practice, the epsilon-greedy policy works by generating a random number at each decision step. If the number is less than ε, the agent selects a random action; otherwise, it follows the optimal action based on its current knowledge. For instance, imagine a robot navigating a maze: initially, it might explore different paths (high ε) but gradually shift to using the shortest known route (lower ε). A common implementation involves starting with a higher ε value to encourage exploration early in training and then reducing ε over time (e.g., via decay schedules) to prioritize exploitation as the agent learns. This balance is critical because pure exploitation might miss better strategies, while pure exploration would waste time on known poor choices.

Developers use epsilon-greedy policies because they are simple to implement and effective in many scenarios. For example, in recommendation systems, ε could determine when to show users new content (exploration) versus proven popular items (exploitation). However, the choice of ε significantly impacts performance: too high, and the agent learns slowly; too low, and it might miss optimal solutions. Advanced variations, like decaying ε or combining it with other exploration strategies (e.g., Upper Confidence Bound), address these trade-offs. Despite its limitations—such as inefficiency in large action spaces—the epsilon-greedy approach remains a foundational method in reinforcement learning due to its clarity and adaptability.