How ArpadZsum works
ArpadZsum is a zero sum rating between 2.0 - 7.0.
Win probability
The higher rated player is assumed more likely to win. The probability of winning on each side adds up to 1.
0.5 apart → 68%
1.0 apart → 82%
1.5 apart → 91%
Team rating = (lower rated partner = 64%) + (higher rated partner × 36%)
Win probability = 1 / (1 + 10^((other team − your team) / 1.4))
1.4 is the "steepness" divisor.
The formula
New rating = old rating + step × margin × win probability
Win probability = what happened minus the win probability. Win = 1, loss = 0.
Step = base for how far one game moves the rating.
Margin = close game vs blowout, from ×0.5 to ×1.5.
An example (doubles)
Team A rates 3.50, Team B rates 4.00.
The ratings give Team A about a 32% win chance — they're the underdog.
Team A wins 11–7. Surprise = 1 − 0.32 = 0.68.
Step 0.10 × margin 0.55 = 0.055. Change = 0.055 × 0.68 = +0.04.
Team A rises to 3.54; Team B drops to 3.96.
Beating a stronger side earns more; losing to them costs little.
Doubles teams
A team's rating factors more to the lower ranked partner: 64% the lower rating, 36% the higher.
