Elo Rating System

Overview

Elo assigns numerical ratings that update after each pairwise comparison. Winner gains points, loser loses points. The amount exchanged depends on expected vs actual outcome. Originally for chess, now used for sports, games, and A/B preference testing. Update runs in O(1) per match.

When to Use

Trigger conditions:

• Ranking items from pairwise comparison data (A vs B outcomes)
• Building competitive rating systems for games or sports
• Crowdsourced quality evaluation through pairwise preferences

When NOT to use:

• When you have absolute scores, not pairwise comparisons (use direct ranking)
• When team dynamics matter more than individual skill (use TrueSkill)

Algorithm