Monday, December 10, 2018

AFL Statistics Series #2: Scoring a Behind – ‘Scoreboard Impact’ or a ‘Missed Opportunity’?

One point for ‘trying’

What is the ‘value’ of a behind in Australian rules? On the scoreboard it is of course one point scored for your team. That’s less than the six points for scoring a goal, but better than no points and potentially the difference between winning and losing.

A behind scored by an individual player is also recorded against the name of that player, as part of their contribution to the team’s score. In fantasy football leagues a behind makes a minor but positive contribution towards a player’s fantasy points total. So while it isn’t as good as a goal, it seems like something at least.

Is it a positive though? Every supporter has known the agony of his or her team losing a match through inaccuracy in kicking for goal. It’s even more agonizing when the players are missing shots for goal that are considered relatively easy.

Shots on goal are hard to come by, and six behinds are needed to obtain as many points as just one goal. Furthermore scoring a behind gives the ball back to the opposition for a kick into play, in contrast to the roughly ‘fifty-fifty’ chance a team has of getting the ball back again from the centre bounce that follows a goal.

With that in mind should we really consider kicking a behind favourably? Should a behind be seen more as ‘impacting the scoreboard’ or a ‘missed opportunity’?

How existing player rating systems credit behinds

As I said in my first post in this series part of thinking here is perhaps to arrive at a new ‘player rating’ system. Both the Australian Football League Fantasy and SuperCoach (Champion Data) ratings give a player a point for scoring a behind. HPN’s PAV system also gives positive credit to a player for any point he scores, though as with the other systems a behind will have only a minor effect on a player’s rating.

In the AFL Player Ratings system however a player can lose rating points for missing a shot at goal. Rating points in this system essentially depend upon how a player’s action affects a team’s expected score. For example, a player taking a shot from 15 metres out would in most cases be expected to score a goal, so scoring a behind instead means the outcome was a lot worse than expected. The ‘penalty’ for missing is less if the player was taking a more difficult shot – say, from 60 metres out.

Kicking a behind could then lead to a net negative effect on the player’s rating. In the extreme if the player who missed the shot on goal gets no credit at all for creating the shot then the player has only hurt his team. On this view, it would be a similar situation to a player undoing the good work of his teammates by kicking the ball to the opposition, and it is well-known that a player loses points for this under the SuperCoach rating system at least.

Reading through the explanation of the AFL Player Ratings system for the first time did alter my view of what a behind was worth. Or maybe it just returned me to a more intuitive state of being a fan in the stands watching a player on my team miss a shot on goal – shaking my head and cursing as he ‘blew’ all the hard work of my team getting the ball up the field for a scoring shot only to get one point out of it.

Attributing a team’s points to player accuracy

Players can help their team score by contributing to the creation of scoring shots, and by taking scoring shots. Crediting players for the former is going to take a bit of work. I think though it’s relatively easy to give credit for the latter.

OK, a player is taking a shot for goal – what is the change in the team’s expected points from him converting or missing the shot? Under the AFL Player Ratings system this depends upon the expected points for the position on the ground that the player takes the shot from. However I only have public data and I don’t know where the shot was taken from. Therefore, let’s define the extra points from scoring shots as following:

Extra points created by player from taking scoring shots = Points scored by player – (League average points per scoring shot, excluding rushed behinds * Scoring shots by player)

In 2018 the league average points per scoring shot, excluding rushed behinds, was 3.85 points. Therefore, if a team creates a scoring shot and I don’t know where on the field the scoring shot was created from, I’m going to assume the value of creating one scoring shot is 3.85 points. These points can be attributed amongst the players who contributed to creating the shot, including perhaps to the player who took the shot itself.

But what is the value of simply taking the shot? That is, let’s ignore the player’s role in creating the shot, the metres gain from kicking to goal, and what happens after the score is kicked. Under the system above each successful shot by a player on goal adds 2.15 points, with the other 3.85 points going to the players that created the shot. On the other hand, if a player misses a shot he can be said to have subtracted 2.85 points from his team’s total.

This simple system does ignore shots at goal that go out on bounds on full, but I cannot get these from public statistics. Probably the bigger weakness though is it does not account for the difficulty of the player’s shots on goal, as the AFL Player Ratings system does. For example, Lance Franklin converted shots on goal in 2018 at about the league average rate. Franklin though is well known for converting longer and more difficult shots than your average forward. (One might also argue that the difficulty of shots varies at a team level – i.e. some teams create better shots than others.)

If we just use this simple system though, which players created the most value last season from scoring shots converted, and which players lost the most value? This will depend upon the accuracy of the player’s shots and his volume of scoring shots. Hence, the top players in terms of extra points added from converting scoring shots in 2018 included leading goalkickers such as Ben Brown, Tom McDonald, Jack Riewoldt, Luke Breust, and Tom Hawkins (see table below).

The ‘worst’ goalkickers do not ‘destroy’ quite as much value as the best goalkickers create (see table below). According to this system Jarryd Lyons lost about 28 points for his team in 2018 through his inaccuracy, less than half of the 63 points Ben Brown created.

The line from ‘good’ to ‘bad’ converter is relatively thin. There is little overlap among the ‘best’ and ‘worst’ converters if the same calculations are done for 2017 (though Ben Brown topped the list in both years), with Christian Petracca even flipping between the two.

On a per game basis the points gained or lost from simply converting scoring shots may seem relatively small. Even Ben Brown is only credited for less than 3 extra points per game from his accuracy. However in a league where a team scores on average 80-90 points per game – meaning that each of a team’s 22 players contributes on average about four points per game – goal accuracy can be quite significant.

It is even more significant in evaluating a player’s contribution to an individual game. A player scoring four or five behinds without scoring a goal could very well obliterate every positive contribution he has made for the game. Few ‘possession chains’ a team or a player is involved in result in scoring shots; ‘wasting’ those that do is significant.

VERDICT: Creating a scoring shot is valuable. Scoring one point rather than six points with that shot is generally a MISSED OPPORTUNITY.

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