Home Ground Advantage in the NRL

Home teams win more than away teams. This is true not only for the NRL, but essentially every sports league around the world.

But most fans know this, that’s why so many people base their weekly tips on who is playing at home, and that’s why most tipping competitions auto-select away teams as a disadvantage when you forget to select your own. But how much time the average fan devote to thinking about why home ground advantage exists?

For most fans, the answer to that question is not much – but they should. Home ground advantage is the spectre that hovers over sport, it has a direct influence over the outcome of games and in some sports can be even more influential on a statistical level than a team’s best player.

For reference, here are the winning percentages for home teams in the NRL since 1999;

In total over the last two decades home teams have won 56.75% of games. When broken down on a points per game basis home ground advantage stands at 3.09 points per game.

But why does this happen? It’s a simple question with a complicated and unclear answer, but with the NRL due to play at least most of their season behind closed doors, it’s about to get a lot clearer.

There is no concrete consensus on the exact factors affecting HGA, but there are several hypotheses speculating on the reasons why home ground advantage exists.

The general feel from fans is that the home crowd must be the largest influencing factor on HGA. The idea being that the noise generated in support of the home team could make them play better, or that boos and insults hurled towards the away team could make them play worse. Sound in theory, however the evidence behind this is conflicting. One of the most cited articles of data running against this theory comes from basketball, where NBA teams hit free-throws at a measly 0.2% better at home compared to on the road. The genius of using free-throws as the reference point is that it’s a static environment, shooting a free-throw is functionally identical given the strict regulations each NBA court is held to – the only difference being the crowd in front of which you are shooting it.

From this some people have hypothesised that the crowd holds limited influence on HGA. Unfortunately this is operating under the assumption that players are the only people who influence games, but there is another group of people who could be influenced by the crowd – the referees.

Home officiating bias exists in almost every team sport relying on officials to make real-time subjective calls and the NRL is no different. In the 2019 season, home teams conceded 0.62 less penalties than away teams.

This is in-line with most other major sports who have found that calls always favour the home teams. Another example from the NBA found that home teams were awarded an additional 1.3 free-throws representing roughly one third of the total home-ground advantage. Another example in the NHL found teams are called for 20% fewer penalties when playing at home. This bias is often attributed to external crowd factors. Referees are only human, and with 20,000 fans all behind one team, you can understand how they might give the home side the benefit of the doubt.

Quantifying how much refereeing bias contributes to overall home field advantage in the NRL is a trickier question. On the surface a penalty is akin to delivering and additional set, but it also comes with a territory advantage assuming the team kicks for touch. If a team already has territory, they’ll have the option of a fresh goal-line set or a penalty goal.

While we don’t have field-based penalty data, we do know that the average points per set in the NRL is roughly 0.54, however that includes all sets. We also know that a set starting on an opponent’s goal line yields roughly 1.1 points per set. Given that any penalty awarded comes with a territory advantage progressing closer to the opponents goal-line, or potentially a fresh goal-line set or direct shot at goal, the actual value of a penalty is significantly more than 0.54 points. As an estimate each penalty could be worth on average somewhere between 0.7-1 points, meaning referring bias could represent somewhere between 0.43-0.62 points contributing roughly 17% of the total 3.09 point home ground advantage.

These wide-spread examples of refereeing bias have led many to believe that the crowd may hold more influence over the officials and not the players as once previously thought. While there is no doubting the presence of bias the fact remains that players do actually perform worse playing away compared to home. Another example from the world of basketball found that NBA teams shoot 1.5% better from 2-point range, 0.5% better from 3-point range, have 0.12 more steals and more offensive rebounds despite missing less shots. None of these statistics are directly influenced by refereeing bias.

It’s often hypothesised that most of this difference can be contributed to environmental factors. Playing away in the NBA almost always involves travel which includes boarding and plane, sleeping in a hotel room and playing in an arena that isn’t familiar. Its often thought that a combination of these factors leads to decreased player performance.

But here is where the NRL goes a long way to debunking this – we have a cluster of teams all located in the same geographic radius, 9 of them actually; Manly-Warringah, Canterbury-Bankstown, Parramatta, Sydney, Penrith, St George-Illawarra, South Sydney, Wests Tigers and Cronulla.

When these teams play each other it all happens within the wider Sydney area. Away players in these games still get to sleep in their own beds, they do not have to board a plane and they get to travel home when the game is over. Some of these games even take place on exactly the same grounds the away team uses as their home ground, the only difference being the composition of the crowd. When South Sydney play Canterbury at ANZ stadium, while they do share that home ground, there are >10,000 season ticketed South Sydney members who have access to the game and visa-versa when it’s a Canterbury home game.

So how do these Sydney-based home teams perform when playing another Sydney club? The answer is better than expected.

The home-ground advantage when two Sydney-based teams play each other is 2.28 points per game, only 0.81 points less than the average HGA.

Despite the benefit of no air-travel and being able to stay in their own beds, Sydney away teams playing in the Sydney area still perform poorly. An argument could be made that the 0.81 point reduction (26%) could be somewhat attributed to the increased ‘away’ support a Sydney team playing away in Sydney would expect in the stadium compared to an out-of-town team. While there could be some environmental influence like playing at an unfamiliar ground such as Leichhardt or Brookvale, its possible that these factors may be overblown in comparison to the crowd.

Home ground advantage has been a sporting mystery for some time, but with the NRL due to forge ahead with a crowd-less May 28 restart, it could become ground zero for gaining a more complete understanding. Teams will still travel to play away games as they would during a normal season with interstate teams like the Broncos, Cowboys and Titans remaining in their respective cities. The only difference being the removed element of the crowd, and how important is the crowd? We’re about to find out.