European researchers using the COVID-19 pandemic as a natural sports experiment have found that while referee bias vanishes in the absence of spectators, home-field advantage — the benefit the home team is thought to gain over the visiting team — persists in European soccer leagues.

The study, published Wednesday in PLOS One, used data from a decade's worth of European soccer matches, from the 2010-2011 season to the 2019-2020 season. Researchers analyzed more than 40,000 matches before and during the pandemic from 10 professional leagues and one amateur league across six countries, including Spain, England, Italy, Germany, Portugal and Turkey. 

"During COVID-19, a lot of matches were played without spectators, and this is a real situation that has never been there before and an experiment we couldn't have done in any other way," Fabian Wunderlich, lead author of the study and research associate at the German Sports University Cologne, told The Academic Times.

Natural experiments like this within the sports industry seldom occur and artificial experiments are not adequate to study questions related to spectator influence, according to the researchers. 

"We can't ask football clubs: 'Can you please go and take 500 matches and exclude spectators,'" Wunderlich said. "The problem is coming up with real experimental setups. You'll never be able to fully replicate, for example, the situation of a real football match. You cannot replicate the psychological pressure, for example, that is present in a real soccer match. You can't do that."

The researchers studied four metrics: disciplinary sanctions, match dominance, market expectation and actual home advantage. They examined the influence of spectators on each of them by splitting their dataset into two types: 36,882 matches played under normal circumstances and 1,006 matches played without spectators because of the COVID-19 pandemic. 

"The advantage of home teams under normal conditions (spectator presence) are reflected in more goals, points, expected points, shots and shots on target, as well as less fouls, yellow cards and red cards [compared to the away team]," the researchers wrote. They measured the effects of spectator absence for all variables by observing decreases or even reversals in these trends. 

Referee bias was measured through disciplinary sanctions, determined by the number of fouls, yellow cards and red cards. Normally, referees sanction the away team more than the home team, according to the researchers; in the absence of spectators, though, this bias vanished. 

"For disciplinary sanctions, no significant differences are present in absence of spectators anymore, except for yellow cards that show a slight home disadvantage with home teams receiving more yellow cards," the researchers wrote. "This is clear evidence that the referee bias completely disappears or is even slightly reversed in empty stadiums."

The researchers' statistical model estimated a home advantage reduction due to spectator absence of 174.3% for fouls, 151% for yellow cards and 106.6% for red cards.

With spectators, home teams received an average of 1.89 yellow cards per match, while away teams received 2.20 yellow cards. Without spectators, home teams' yellow card sanctions increased to an average of 2.21 yellow cards per match, while the away team saw a decrease to 2.06. 

Likewise, with spectators, home teams received an average of .09 red cards per match, while away teams received .12. Without spectators, home teams' red card sanctions slightly increased to .12, while away teams saw a decrease to .11. 

"Referee bias exists, and it really seems to be induced by spectators," Wunderlich said. 

"What is usually mentioned in the literature is that the crowd in the stadium is assumed to put social pressure on the referees," he explained. "This social pressure is going to evoke a change, either consciously or probably unconsciously, in the referee's behavior."

The researchers also studied spectators' influence on home teams' typical dominance over the away teams during a match — that is, the home team's "match dominance," measured by "shots" and "shots on target." 

"Shots" are any attempts to score a goal, while "shots on target" denote a goal attempt that would have gone into the net had it not been stopped by a goalkeeper or another player who is the last line of defense. 

The researchers found a statistically significant decrease in home teams' shots and shots on target in the absence of spectators, suggesting an overall decrease in home teams' match dominance. The researchers' statistical model estimated a home advantage reduction due to spectator absence of 57.6% for shots and 55.1% for shots on target. 

With spectators, home teams had an average of 4.82 shots on target per match, while away teams had 3.88 shots on target per match. Without spectators, home teams' shots on target reduced to 4.25, while away teams' shots on target diminished by .01, resulting in 3.87 per match.

While match dominance is a facet of home advantage, since more shots and shots on target might translate into more goals, it is not the ultimate measure of home advantage in this study. Rather, home advantage was measured by actual goals scored and points awarded.

Goals refer to the number of goals scored by each team, while points refer to the number of points awarded in the overall league — 3 points for the winning team, 0 points for the losing team and 1 point for each draw. 

The researchers found that home teams' goals and points slightly decreased in the absence of spectators, but that decrease was not statistically significant, suggesting that spectators are not the main driving factor behind the home advantage. 

With spectators, home teams scored 1.50 goals on average per match, compared to 1.14 goals scored by away teams. Without spectators, this only slightly diminished for home teams, as they scored 1.47 goals on average per match, while away teams scored 1.24 goals per match. 

Moreover, with spectators, home teams gained 1.62 points on average per match while away teams gained 1.11 points. Without spectators, the home teams' average points per match were reduced to 1.54, and away teams' points increased to 1.21. 

Overall, home teams continued to see more wins than away teams. Per 100 matches on average, the ratio of home team win to draw to away team win with spectators was 45:27:28, respectively. Without spectators, the ratio slightly diminished for home teams to 43:25:32.

"This was the most surprising part to us — that we saw a decrease in home advantage, but it's not significant," Wunderlich said. "There is still home advantage even when there aren't spectators. The decrease in home advantage based on our estimates, if it exists at all, is way smaller than what you could expect."

"This gives us insights on the reasons of home advantage because we can be sure now that spectators are just a minor effect on the home advantage," Wunderlich said. "Spectators are not as important as a lot of people think."

Additionally, researchers investigated how the absence of spectators impacted expectations in the sports betting market. The market expectation was deduced from betting odds, which might offer a glance at how the sports betting market is assessing home advantage with and without spectators, according to the researchers. 

"Sports betting or betting odds have been found to be an excellent predictor of sports," Wunderlich said. "If you want to know who's winning the match, just look at the betting odds and you have a very good estimation based on all relevant factors, including who's playing at home; they are less subject to randomness."

Researchers found "a significant decrease of a home advantage by one-third" on expected points based on betting odds, Wunderlich said, adding that this "is an indication that the effects we see might actually be something systematic and not something random."

The results of this study suggest there are other factors at play that are contributing to home advantage, according to the researchers, though they said it was striking that decreases in home team match dominance and referee bias had limited effect on the actual home advantage that persisted even without spectators.

On referee bias's limited effect, "We can only speculate about the reason" Wunderlich said. "Probably the mechanism behind it is just that increased sanctioning doesn't contribute that much to the results. In my opinion, this might be the case because the only sanction that really changes a match dramatically are red cards, and those don't occur too frequently."

As for match dominance's limited effect, "There are a lot more shots than there are goals, so it's easier to find a statistically significant result here," Wunderlich said. Moreover, "It's possible that teams in the presence of spectators attempt more shots on goal without being more successful. This is a theory that could explain the data that we see, but we do not have full evidence of it."

Wunderlich emphasized the need to continue to study the effects of the spectator phenomenon in sports and the need to study other factors that may be contributing to home advantage. 

"In terms of research, it's gonna be really exciting to see spectators returned to the stadiums," he said. "Once the spectators fully return, we can get a better picture on this. This is something that is still not fully clear and this is something that needs a lot of more research to really understand what drives the home advantage in terms of the practical side of soccer."

As for the implications of this study, Wunderlich said, "Referees and officials need to be aware that they can be biased, and they need to be aware that they can be affected by social pressure. They need to make sure to implement special training for referees to not be biased against the away team."

The study "How does spectator presence affect football? Home advantage remains in European top-class football matches played without spectators during the COVID-19 pandemic," published Wednesday, March 31 in PLOS One, was co-authored by Fabian Wunderlich, Robert Rein and Daniel Memmert, German Sports University Cologne; and Matthias Weigelt, University of Paderborn.