Difference-makers: Can a +/- rating work in the AFL?

When you think of the players who are most valuable to their team, you might think of stars like Christian Petracca and his impact at Melbourne, Charlie Curnow's goalkicking prowess at the Blues, or Oscar Allen's value to the struggling Eagles.

On the ESPN Footy Podcast last week, Christian Joly from Champion Data brought to the table a fun stat after a trend was pointed out regarding Hawthorn's performances with James Sicily in the side as opposed to him out of the side. One of the Hawks' most impressive performances came in Round 11 when they shocked the then-red hot Saints at Marvel by 10 points.

Sicily was a clear best-on-ground, picking up 43 disposals and 16 marks. The next week, he didn't play; he was suspended and the Hawks got trounced by Port Adelaide by 55.

The following week, Sicily returned, racking up 32 touches and 12 marks against the Lions, and, again, the Hawks pulled off a huge upset. Hawthorn then had the bye, and Sicily missed the next three matches through suspension again... three losses, by 67, 60, and 13 points followed. Sicily came back into the team, and the Hawks won again, albeit against North Melbourne.

The eye test tells you one thing -- that he's clearly Hawthorn's most valuable player -- but it did beg the question, is it Sicily who is the league's most important player to their team?

"When he's been on the field or playing for the Hawks, they're -8.6 points per 100 minutes played. So they're still getting outscored when he's on the field, but they're a young team sitting bottom three," Joly said on the podcast. "When he's either on the bench or doesn't play, [the Hawks] are -37 points per 100 minutes, so it's a four goal difference with him either being out there or not."

It got us thinking about a similar stat we often see in the NBA: the +/- (or, plus/minus). Basically, a score-based metric which tallies how much a team's net score moves while a player is on or off the court.

For instance, if a player is on the floor when their team puts on a run of eight straight points, their +/- is +8. Similarly, if the player is on the floor later in the game, and they're outscored by three points in that time, the overall +/- for that player moves to +5.

While not as illustrative as the impact a player can have on an NBA floor given there are just five per team in play at any one time as opposed to 18, nonetheless, it's interesting to look at a sample from the AFL to see which teams are impacted by certain players either being or not being on the ground.

But it's not as simple as the way the NBA records the stats, given how interchanges work in footy, and how some players including midfielders play up to 95% game time (some key position players don't come off at all).

For the query, we sorted the competition by the top 350 players for minutes played, then filtered those players for anyone to miss at least 500 minutes of game-time this season (in order to give a big enough sample size for players who have missed games, or who have spent significant time on the bench), to give us a greater understanding of a team without a player as opposed to with a player on the park.

That gave us a sample size of 215 players, and from there we looked at the team's points differential per 100 minutes played while each player was on the ground, against the points differential per 100 minutes played when the player was off the ground (or out of the side).

So can a slightly tweaked version of a +/- rating work in the AFL?

Officially, between Rounds 1 and 19, the Hawks were -8.59 points per 100 minutes with Sicily on the park, and -36.92 without him, giving him a +/- of +28.33 in 2023.

But he's not the most impactful player according to Champion Data.

In the same time span (Round 1 through 19 in 2023), the Eagles' saving grace has been fullback Tom Barrass. When Barrass is on the field, the Eagles are -39.32 points per 100 minutes. Without him? A staggering -75.97, meaning Barrass has a league-leading +/- differential of +36.65 points per 100 minutes.

Barrass is one of three backmen to rank in the top four in this stat, with Essendon's Jayden Laverde (the Bombers are +11.76 points per 100 minutes with him on the field, and -24.82 with him off, for a differential of +36.58) and Carlton's Brodie Kemp (+26.13, -7.51, for a differential of +33.64) nestled either side of Hawks forward Tyler Brockman (+0.08, -35.82, for a differential of +35.90). Kemp has maintained his spot in Carlton's side as they've started to improve, having played 11 games in 2023 for seven wins and four losses.

In fact, Sicily ranks ninth out of those 215 players for +/- impact.

On the other side of the ledger? Well, the player with the worst +/- differential is Geelong's Jed Bews. With Bews, the Cats are -5.19 points per 100 minutes, but without him, are +35.84, for a differential of -41.03.

For some additional context, the Cats are 9-9-1 on the season, but when Bews has played, the record is 5-7-1, lending itself to a slightly better differential when he's been out of the side.

Lewis Young and Jesse Motlop, mainstays during Carlton's run of losses, are second and third, respectively (differentials of -40.68 and -40.23), while Patrick Cripps also rates as the fifth in the bottom 20; the Blues are +1.99 with the skipper on the field, but +35.06 without him, for a differential of -33.07. It's worth noting Cripps missed the Blues' massive win over the Eagles in Round 19.

What can be drawn from all this? It's perhaps that individual players probably don't have as big an impact as 'team form' does. Highlighted earlier, the difference between a five-man floor in the NBA and an 18-man team in the AFL is not to be underestimated, and being fit at the right time (that is, when your team is hot) probably helps a player's +/- differential - just look at the contrasting cases of Young and Kemp at the Blues.

It's also difficult to make solid conclusions from the numbers considering some players (key backs in particular) might play 98% of each game, making a 'time on, time off' stat inconclusive due to small sample sizes and the nature and timings of interchanges in footy.

Then again, there are some players the 'eye test' supports having such positive differentials, like Sicily, Barrass, or Dion Prestia for the Tigers.

Let us know what you think of the podcast segment by Tweeting us @footytips.