Multi-armed bandit

A multi-armed bandit is a testing method that shifts traffic towards the better-performing variant while the test is still running, instead of holding a fixed split until the end. It suits short-lived decisions — subject lines, offer ordering, content slots — where the cost of sending the loser is real money. The trade-off is measurement: because allocation changes over time, a bandit delivers a good outcome but a weaker estimate of how much better each variant actually was.