Have you ever won five games in a row and felt like you could win ten more? Or maybe you lost 5 five and you just kept losing after that? Most people that have played sports will recognize this. Being “in form” seems to have a large impact on whether we will win the next game or not. Because of this it is one of the most discussed topics in football. However, it is very elusive and to this day we still do not know if form is even real or not.
Form
Form is typically described as the state of a player or team’s performance. A player is said to be in good form if he has scored lots of goals in the last couple of games and a team is in bad form if it has lost its last few games. This is already very subjective: different individuals can have different opinions about the performance of a certain player. Furthermore, can you really say a team is in bad form when it has lost its last few games to teams much stronger than itself? This just shows the first few problems when defining the impact of form on football
Now there is the problem of quantifying form. As there are many factors which contribute to the chance of winning the next match, measuring the effect of form is very hard. Nevertheless, some data scientists have studied the effect of form on scoring goals. One study found that there was no correlation between scoring a goal in the previous game and scoring one in the next. This seems to suggest that form has no effect on the chance of scoring at all.
Recency and negativity bias
One Cambridge article looked at the correlation between form and odds set by bookkeepers. The results were quite interesting: it showed that bookkeepers consistently set wrong odds when teams were in really good or bad form, especially the last game was important to bookkeepers. Here they showed that bookkeepers had a recency bias, where bookkeepers valued recent games more than they should. They also showed that odds were especially extreme when a team was in bad form, which means bookkeepers also had a negativity bias. In the end bookkeepers just set their odds such that they will always make a profit, hence these odds are largely set based on public opinion. However this again shows that how we perceive form will impact the next game is wrong.
Regression towards the mean
The chances for a single player to have 5 straight games which are better than usual are pretty slim. However, when you realise there are hundreds of players in each league there will almost always be a few with such hot streaks. Imagine a player has a 50% chance of playing a good game. Now the chance of having 5 good games is: $0,5^5$ which is pretty low. However the chance that none of the approximately 200 players in a league have a good streak is $(1-0,5^5)^{200}$, which is extremely small. This shows that simply because of chance there will almost always be players in form. After such a streak it is more likely the player will have a more “normal” result than having this exceptional streak again. This is known as regression towards the mean.
There obviously are players that keep outperforming even after this first streak. However this usually means the player simply got better or circumstances have changed. Opinions differ on whether this means a player is still in form or just performing on his new “normal” level.
Conclusion
In the end you should not rely on form too much. Even though you can look at recent games to try and predict the future outcome, there is no proof form has any impact on this. Furthermore, when betting it might even work better to bet against the current form of the team given how bookkeepers set their odds.
There are many more interesting articles about statistics in football, like this article about home advantage in football.