Regret measures the cumulative reward lost by choosing actions instead of always selecting the best available action. In multi-armed bandits, it exposes the cost of exploration and poor exploitation. section 12.1.6
Three slot machines over 90 trials
Suppose three machines have mean payoffs of 8, 10, and 5. Choosing each machine 30 times produces 690 expected reward, while always choosing machine 2 produces 900. Regret is therefore 900 − 690 = 210. section 12.1.6
Compare strategies by accumulating per-trial regret:
- Explore-only samples machines randomly, while exploit-only greedy commits to the currently highest-reward choice. section 12.1.6


