Do goals stimulate goals? / by Matthias Kullowatz

By Matthias Kullowatz (@MattyAnselmo)

I've heard it said before that a soccer team is most vulnerable after a goal has been scored. My coaches often said this, anyway. Perhaps it was just to keep us focused after we'd experienced the euphoria of scoring or the letdown of conceding. It turns out, there is some support for this notion from the 2014 MLS season. To the results! 

1) A goal is more likely to be scored in a five-minute segment if a goal was just scored during the last five-minute segment.

First, I should say that I controlled for the teams' abilities using season expected goals data, and I controlled for the gamestate as well, since there are fewer goals typically scored in zero gamestates. Once controls were in place, I found that if a goal had been scored by either team in last five-minute segment, the chances of another goal being scored in the next five-minute segment increased from about 15% to nearly 18%.  Put another way, after a recent goal the average goals scored in next five-minute segment increased by nearly 0.04, equivalent to about 0.70 goals over a whole game.

This isn't an obvious uptick in scoring. You probably wouldn't notice even if you watched a lot of games, but the effects of a recent goal are also not nothing. The game appears to open up a bit on average after a recent goal. 

2) The team that most recently scored is more likely to score again (than its typical scoring rate would suggest).

Breaking the first hypothesis down further, we actually see that the team most likely to score in the next five-minute segment is the team that just scored.* The chances of a team scoring in the next five minutes--whether it be the away team or the home team--are increased by 3 or 4 percent if that team scored recently. Chances increase from 6% to 9% for the away team and from 9% to 13% for the home team.

Typical sports fans may say "duh" because the existence of momentum in sports is a common belief. However, momentum is still very much a point of controversy among statisticians across all sports. I would say about the only thing we agree on is that the effects of psychological momentum are smaller than the common fan would believe, and perhaps even negligible in many scenarios. That's why this finding surprised me, especially when we consider that the team that just scored must then relinquish possession.

These results may not apply to a youth soccer team, or even a professional team from another league. But in MLS, there is an average effect on scoring, which is not necessarily negligible, that comes from a recent goal being scored.

Comments are welcome, especially if you can think of a way to further control the scenarios and weed out any biases in the observational data.

*Of course, the team that just scored is probably the better team. But that's why a control was put in place for overall team ability. What isn't controlled for is team ability on that day (due to injuries and what not).