A/B Testing vs Multi-Armed Bandit

Example

Testing 3 landing page headlines:

• A/B → 33% traffic each for a week.
• Bandit → By day 2, more traffic flows to the top performer, yielding higher total conversions.

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Why Is It Important

Choosing the wrong approach means wasted traffic or missed opportunities. A/B gives credibility and governance; bandits give efficiency and real-time optimization.

Interview Tips

• Define both clearly.
• Link method to business goals (learning vs earning).
• Mention metrics: lift, regret, horizon, stationarity.
• Propose a hybrid: A/B for initial testing, then bandit for exploitation.

Trade-offs

• A/B: Reliable estimates, slower learning, higher cost.
• Bandit: Faster optimization, higher short-term reward, but less stable effect sizes.

Pitfalls

• Stopping A/B too early.
• Misinterpreting bandit outcomes as unbiased lift.
• Ignoring delayed/non-stationary rewards.
• Optimizing the wrong metric (CTR vs revenue).