Ghost games in the Bundesliga have never been done before. But what does the exclusion of viewers actually mean for the games? Some even fear a distortion of competition because home teams without their own fans could be disadvantaged in the back. Daniel Memmert, professor at the Institute for Training Science and Sports Informatics at the German Sport University Cologne, can calm down.

ZEIT ONLINE: Mr. Memmert, do the hosts of ghost games now fear for their home advantage?

Daniel Memmert: First of all: There is actually a home advantage, it can be proven, but it has decreased in recent years. In the major European leagues, around 50 percent of home games are won on average. But he has less to do with the spectators in the stadium. The influence of the audience is systematically overestimated.

ZEIT ONLINE: Really?

Memmert: New studies show: the absolute number of spectators, the occupancy rate, but also the behavior of the spectators are hardly or hardly at all related to the outcome of the game. This was particularly impressive to see in Italy in the 2006/2007 season.

ZEIT ONLINE: Of all things.

Memmert: Yes, actually. At that time, 20 meetings were held without spectators. Back then for security reasons. A comparison between these games and those that were held with the audience showed: nothing. It was irrelevant for the result whether the stadium was empty or full.

ZEIT ONLINE: 20 games may sound like a small sample.

Memmert: Right. It is not robust. There is another study for this with a rather large sample: More than 10,000 games were watched at the lowest level, i.e. at the level of district class A. There is also a certain home advantage there, although these were almost all ghost games because maybe there 30 spectators come. We conclude that the viewers are at least not directly responsible for the home advantage.

ZEIT ONLINE: How is it to be explained then?

Memmert: With familiarity with your own sports facility, the lawn, the stadium, the cabin. Even the people who greet and clap you on the way from the cabin to the square could play a role. The players seem to feel more comfortable at home, everything is familiar, you know your way around. This is the same for all of us when we get into our four walls in the evening. Unfortunately, this assumption is difficult to prove empirically.

ZEIT ONLINE: Are there any other reliable reasons?

Memmert: Another option would be: by the referee. Using a very large data set of 5,000 Bundesliga matches, we were able to show that more yellow cards are given to the away team than to the home team. Half a card more, if on average both teams would get two cards each. It's a big effect. This effect became even greater when there was a game in a stadium where the spectators were sitting very close to the edge of the field.

ZEIT ONLINE: So the viewers have influence?

Memmert: Yes, indirectly via the referee. We tested this hypothesis in an experiment. We invited referees and played foul situations for them. Once with sound, once without sound. With noise in the background, the referees showed significantly more yellow cards. The referees seem to pick up the noises from the audience and unconsciously include them in their evaluation.