Hi all, I've been wanting to make a post about Player Ratings (PRs) system for a little while given discussion is dry this time of year and I'm a stats nerd. I think it's a hugely useful statistic for measuring direct player impact on games, and even though it has blind spots and limitations it's still arguably the best single-number metric yet devised for the game.

In this post I'm hoping to give an overview of the system through how it's calculated, what it does and doesn't capture, its quirks and how best to use it as a tool for analyzing the game. Despite being famously opaque, PRs are a system that Champion Data (CD) have published a full explainer document on. It's a little bit dense and a touch self-serving though, so I thought I'd make my own explainer to try and generate discussion.

The Short Version

The shortest explanation for PRs is that it's a measurement of direct impact each player has on a game through their actions that affect either the ball or ball-carrier, expressed as scoreboard impact.

To get this measurement three factors are considered for each action:

• The location of the ball on the ground before and after the action
• The situation before and after the action
• The difference in Next Expected Score (NES) value before and after each action

Actions that increase a player's team's chances of scoring next (e.g., taking possession, moving the ball forward) give positive ratings and actions that decrease those chances (e.g., turning it over, missing shots at goal) give negative ratings. The PR each player gets at the end of a game is the sum of these ratings.

Highlights

To give a feel for what really good games and seasons look like on the PR system here are a few examples:

• The highest rated player in the Grand Final was Lachie Neale with 21.5
• The only players to average 20+ in a season with min. 15 games are Gary Ablett Jr (2012-14), Patrick Dangerfield (2016-17) and Dustin Martin (2017)

How it's Calculated

As mentioned above CD use a NES model to get a value for every area on the ground based on the situation, which are typically ways a player can take possession.

The six situations for a ball to be in are:

• Set Position (mark, free kick)
• Uncontested Possession (ground-ball gather under no pressure, or handball receive with no pressure)
• Looseball Get (ground-ball gather with pressure but no physical contact from opposition)
• Hardball Get (ground-ball gather with physical contact from opposition)
• Ground Contest (the ball on ground level up for grabs by either team)
• Marking Contest (the ball in the air up for grabs by either team)

A visual guide for each situation below:

CD map of each area on the ground's NES value by situation

These values are derived from play-by-play tracking CD have been employing since 2007. They start by measuring distance to goal in 20-metre increments, trace what the next score in the game was from every similar event (negative values assigned to the opposition scoring), and use that to create a NES value. The formula used is:

NES = (team goal% x 6) + (team behind% x 1) + (opp. goal% x -6) + (opp. behind% x -1)

Using one of the examples in CD's own document, we'll take marks 40-60m from goal. These result in a team goal 52.1% of the time, a behind 42.3% of the time, an opposition goal 4.2% of the time, and an opposition goal 1.4% of the time.

NES = (52.1% x 6) + (42.3% x 1) + (4.2% x -6) + (1.4% x -1) = 3.13 + 0.43 -0.25 - 0.01 = 3.3

Disposals

In the example above, if a player were to kick a goal from there, they would take the NES from 3.3 to 6 and receive a rating of +2.7 for the action. Similarly, if they kicked a behind it would go from 3.3 to 1 and give them a rating of -2.3.

All disposals are measured this way, with the difference between the NES at the moment of disposal and where it ends up credited to the disposing player. In instances where two players both contribute to the change, such as a mark on the lead from a teammate or an opposition interception, they are each given half the value of the change in rating.

For example: a starting NES of 1.5 that ends up as 4.5 because of a mark on the lead from a teammate gives both kicking and marking player +1.5 rating points. Similarly, a turnover that goes from 1.5 to 0.0 gives -0.75 to the kicking player and +0.75 to the opposition player.

When pressure is being applied to a disposing player, there are greater rewards for effective disposals and lower penalties for ineffective disposals, reflected in a lower starting NES for each event. Using the same numbers above, here's a table showing approximate levels of change in starting NES based on pressure applied for a kick that leads to a teammate's uncontested mark:

Possessions

Winning possession works the same way, but is calculated separately from the result of a disposal. Ratings here only looks at the situation before and after possession is taken, with more importance assigned to winning a ball in dispute (either aerially or at ground level) or intercepting from the opposition. No value is given to winning the ball in an uncontested mark or handball receive as the change is credited to the disposing player (with the only exception being a mark on the lead).

Defensive Actions

While the vast majority of ratings points in this model comes from gaining possession and efficiently disposing of the ball, changes in NES are also credited to defensive actions, such as spoils, tackles and pressure acts.

• Spoils: the spoiling player is credited with the change in rating from a marking contest to a contested groundball. If the spoil goes directly to a teammate, they also receive half the points received from the teammate's gather.
• Tackles: the tackling player is credited with the difference in rating from the opposition being in possession, to either a stoppage or free kick.
• Pressure acts: when the NES value at the time of disposal was lower than when the player took possession, that change is credited to the player/s applying pressure

Other Actions

Aside from those mentioned above, the only other actions to receive ratings points are free kicks and hit outs, which are both shared between the two players involved.

For free kicks half of the change in value is applied to the player receiving the free kick, and half to the player giving it away. From a forward's perspective, if they were in a marking contest with a NES value of +2 and won a free taking it to +4, they would receive +1 for this change. If they gave a free away taking the NES value to +1, they would receive -0.5. Similarly, the defender in these situations would see it go from -2 to -4 (-1 change in rating) or -2 to -1 (+0.5 change in rating).

Hit outs only affect a change in rating if a player on either team gather's the ball directly from a hit out. If the hit out goes directly to a teammate, the ruck receives 67% of the change in rating and the midfielder 33% (e.g, 0 to +1.5 would give the ruck +1 and the mid +0.5). If an opposition player intercepts the hit out, they are given 67% of the change in rating and the ruck receives 33% (e.g., 0 to -1.5 would give -0.5 to the ruck and +1 to the oppo midfielder).

What It's Good For and What It's Not

If you've followed footy for any amount of time you've almost certainly opened up player stats on the app and had a read through the box score. We all intuitively understand that a player who has 30 disposals, 8 contested possessions and two goals has had a pretty good game, but it's hard to accurately compare that to a teammate who had something like 22 disposals, 18 contested possessions and 10 tackles.

The value of PRs is collating all of these direct actions, giving them a weighted value based on ground location and efficiency in use, and showing how impactful that was in terms of the scoreboard. It shouldn't be used as your only way to analyse a game, but it is the best single-number measurement we currently have available.

It's certainly not the final decider on who the best player in the game across any stretch of time as CD typically present it as. It has a huge number of blind spots and biases, and because it's only measuring what it's measuring you do need to use your own intuition and nuance when applying it.

Limitations and Other Quirks

It's biggest and most obvious limitation is that it only measures direct actions players make. It heavily favours players who do everything by themselves (GAJ, Bont) and is less favourable on players who do heavy amounts of grunt work (Rowell, Libba) that unlocks the rest of the team, or players who prowl on the outside waiting for loose balls and handball receives (Gulden, Treloar).

As well as this it's also focused largely on accumulating actions, and has no sense of positional value. This obviously reflects poorly on KPPs, and leads to quirks like Charlie Curnow being the highest rated Key Forward but only the 61st best player overall, or Nick Blakey receiving more ratings points than Harris Andrews. Champion Data have attempted to address these with statistics like 100X, which measure how many ratings points a player gets above the positional average for every 100 minutes in that position (or in defender's case, how much they keep their opponent below their average per 100 minutes in each matchup). Unfortunately none of this is released to the public, and so we just have to use our own intuition when it comes to positional value and defensive efforts.

Conclusion

In essence Player Ratings are extremely good at doing what it's designed to do, but not anything outside of that. If you have an understanding of what that is and what they mean they're a great tool to have in the toolbox. There's obviously no replacement for watching the game, but it's impractical to watch every second of every game or hold all of the information in your head without the use of stats. Player Ratings, to my mind, manage to strike the balance of providing useful advanced information without being so abstract as to be totally removed from what's happening on the field.