AFL Statistics Series #4: Gaining Territory to Score

Back – way back – in my first post in this series, I said that one of the most important measures for an AFL team to win, outside of points for, was its metres gained.

As with points for, there is something obvious about this: a team that scores more is usually going to have more metres gained than its opponent. It is difficult to directly break down a player’s ‘points for’ though, outside of the goals and behinds they directly kicked, and most people would not think that is how to measure a player’s entire impact on their team scoring. Unless the player took it out of the centre themselves someone had to get the ball down to the player that scored.

We do though have a measure for the metres each player gains, at least recently, and with gaining metres being a necessary condition to a team scoring it may help us to better break down and estimate what a player contributes to their team’s score. I expect this to be a main component of my system of player value that I outlined last month.

Metres gained, as I said, are only a recent measure, available on Footywire back to 2015. However, another part of what I will examine below is that it is more or less an extension of older measures that have been with us longer and are more accepted and understood, like inside 50s. That is, valuing inside 50s is similar to valuing metres gained, and vice versa.

I stress here though that I am not heading towards the conclusion that the most valuable players are those that gain the most metres. For a player to gain metres they either need to win the ball themselves, or someone needs to win it for them. When we get to examining the value of winning the ball though, we should be able to express that too in terms of how it contributes to a team gaining metres, and ultimately generating scoring shots.  

Gaining metres leads to scoring – unless you don’t keep the ball

First off: how much does gaining metres tend to lead to more scoring? There is clearly a positive relationship (see chart below), but not an acute one. For example, the Tigers gained by far the most metres in the 2020 home and away season, but were fourth in scoring shots (and also in goals).

Gaining metres by itself of course does not necessarily result in scoring – some teams may play ‘aerial ping pong’, kicking it back and forth to each other without anything crossing the goal line. The next relationship to consider then is that between metres gained per turnover and scoring shots (see chart below – we could also add in team possessions that result in stoppages, but it doesn’t change a lot the point I’m making for now).

Now that looks a bit stronger. The Tigers had more metres gained, but also more turnovers than other top sides, which stifled their scoring capacity relative to the territory they gained. (They also create more opposition turnovers as well, but we’ll get into that when we look at the ‘points against’ side of things.)

Compare this to, say, the relationship between scoring shots and contested possessions, given that contested possessions are often cited as an important factor in winning (see chart below). The relationship is, basically by definition, not as direct as metres gained – teams can advance the ball towards goal through either contested or uncontested possessions, and they differ in the number of possessions they take to get downfield. That is not to say that contested possessions do not matter, but it is within the context of how they ultimately help a team to gain territory that we may get a more accurate overall picture of how they influence a result.

Again, while from a team perspective gaining metres without turning the ball over is what leads to scoring, that doesn’t necessarily mean it’s that simple to measure a player’s value. The players that have high average metres gained per game and high metres gained per turnover tend to be general rebounding defenders like Daniel Rich, Tom Stewart, Shannon Hurn, Jordan Ridley, and Nick Vlastuin. These would still be among the better players in the game, for their ability to kick long and find targets, but they would also often be assisted by teammates, and we’d also need to consider this to get a better picture of their value.

Approximating metres gained with other ‘territory’ statistics

As said above, metres gained are only available to us (I think) back to 2015, but there are other statistics that have been around longer that also capture the concept of gaining territory. Inside 50s and rebound 50s – which measure how many times a player moves the ball int their forward 50 arc, and out of their defensive 50 arc respectively, are available on AFL Tables from the 1998 AFL season onwards.

Between inside 50s, rebound 50s, and scoring shots, we have three measures of when a player has moved the ball past specific points on the field. How well do these relate to metres gained? Fairly well, with players that have high average metres gained tending to have high average totals if we combine their inside 50s, rebound 50s and scoring shots (see chart below). For AFL players who had a minimum of five games in 2020, their estimated average metres gained per game rose on average by:

-          about 60 metres for each increase of one in their average inside 50s;

-          about 50 metres for rebound 50s,

-        about 20 metres for goals (noting that a player can record both a score and an inside 50 in the same kick), and

-          about 30 metres for behinds, also keeping in mind the opportunity cost of missing the goals. 

This approximate estimate of metres gained is within plus or minus 30 average metres per game for about 80 per cent of players. (Of course, I don’t know exactly how metres gained are calculated, so one possibility is that what I’ve just done is re-create something close to the method by which they are worked out.) Without looking in detail at the position of players, it does seem to underestimate metres gained more for ‘outside’ players and wings, and overestimate them more for ‘inside’ midfielders and rucks. However, that doesn’t necessarily count too much against this method of approximating metres – the estimate may actually be a better measure of player value than the ‘raw’ measure, since getting the ball inside the 50 metre arc could carry slightly more value than just the ‘raw’ metres gained would suggest.

What this method does miss though is movement of the ball by players between the 50 metre arcs. Can metres gained also be fairly well estimated from the ways of moving the ball – i.e. kicks, handballs, marks, and hit outs? Yes (see chart below), though as it turns out, at a player average level at least, only kicks contribute positively to the estimate at about 30 average metres gained for each kick. Marks in particular contribute negatively (minus 10), perhaps indicating that players that take more marks are more likely to do sideways or even backwards kicks, that is, chip the ball around the backline.

What this means though is we have come full circle from the more ‘wonky’ measure of metres gained all the way back to a player’s ‘basic’ statistics. A player’s kicks were always a way of indicating to us how much territory a player was gaining. Metres gained may (I believe) measure this more precisely, but as a concept it’s just taking us back to how often and by how much a player is responsible for moving the ball towards the goal posts.

Probably the next part will be to work out which players assist in or set up the gaining of metres. For that, we’ll look more closely at ‘possession chains’ to see which types of possessions are more likely to lead to certain other types.