Hello Elo

Beyond wins, losses, RPI and to a better determination of team strength and quality

What is Elo?
Elo is a rating system for calculating the strength of teams by looking at historical outcomes. Originally created for Chess, and named after its creator Arpad Elo, it has since been applied across a range of sports. We are not the first to apply it to field hockey data - the current FIH world rankings is Elo inspired, Bolster Field Hockey published Elo rankings in 2022, and more recently Allison Keefe has. However, Elo can be tweaked by the creator, through incorporating or excluding different factors, and adjusting the weight assigned to its components and so there is no single, universal Elo model.

Our approach

Each team starts at 1500. Before every game, the model compares the two teams’ current ratings to estimate each team’s win probability. After the game, ratings move up or down based on whether the result was better or worse than expected.
Expected win probability = 1 / (1 + 10^((Opponent Elo - Team Elo) / 400))

The model uses a base K = 80, meaning ratings are fairly responsive to new results. We also include a margin-of-victory adjustment, capped at 2.25, so convincing wins matter more than narrow wins, but extreme scorelines do not overly distort ratings.
Rating change = K × margin multiplier × (actual result - expected result)

Between seasons, ratings regress toward 1500.
New season Elo = 1500 + 0.75 × (Previous Elo - 1500)
This keeps some program strength from year to year while recognizing roster turnover, graduation, transfers, and coaching changes.

Why is this better than RPI or other approaches?

Better depends on how the metrics are applied. We can think of RPI as a measure of a teams résumé rather than as a predictive rating or direct measure of team quality, such as Elo. A teams schedule accounts for 75% of the RPI calculation:

RPI = 0.25*(team win %) + 0.50*(opponents’ win %) + 0.25*(opponents’ opponents’ win %)

This can favor teams that play in strong conferences and create situations in which beating a weak opponent may be less beneficial for tournament selection than not playing the game at all. RPI also ignores margin of victory, treating a 10–0 win the same as a 1–0 win. Within a team’s own winning-percentage component, every victory counts equally; the quality of the opponent is reflected only indirectly through the schedule-strength components.

For most of the data we collect - penalty corner conversion, for example - we are more interested in how it relates to underlying team strength than to a team’s résumé. Because Elo adjusts each result for opponent strength, gives greater weight to unexpected outcomes, can incorporate context such as margin of victory, and produces estimated win probabilities, it provides a more suitable benchmark than RPI.

How can we use Elo?

Team ratings
As noted, we can use it to judge team strength based on results and opponent quality - the higher the Elo the stronger the team. Unfortunately, Elo is not listed as an NCAA tournament selection criteria, and “Coaches’ polls and/or any other outside polls or rankings are not used as a selection criterion by the field hockey committee” either. It’s possible then that team with a higher RPI but a lower Elo than another team could get selected, even though the Elo model considers the omitted team to be more likely to win a neutral site match up. In other words, the selection process may favor the stronger résumé rather than the stronger team. A good record does not necessarily indicate a high-quality team, just as a weaker record does not necessarily indicate a poor one. Without considering underlying team strength or expected win probability, the selection process risks rewarding results without fully accounting for how likely those results were.

Coach impact
We can examine changes in a team’s Elo rating before and after a coaching change to assess whether performance improved or declined. While this does not isolate the coach’s influence from factors such as roster turnover, it provides an opponent-adjusted measure that could assist athletic directors with hiring decisions and coaches during contract negotiations. We’d love to help you get that raise!

Schedule planning
Theres a whole heap of possibilities here, from targeting teams strong enough to improve your résumé but who still represent a realistic opportunity for a win, to finding teams with suitable Elo ratings but who are near you to reduce the impact and cost of travel.