After Wigan lost to Manchester City last March, Wigan manager Roberto Martinez told the press that City stopped Wigan's momentum with fouls: "City used their experience," the Wigan manager said. "There were a lot of technical fouls every time we got into attack situations. Micah Richards should have seen a straight red for his tackle on Tom Cleverley, but the overall number of fouls shows how the game went."
Looking at the facts of the match, it's hard to quibble with Martinez's assessment. For much of the match Wigan were the only side with any ideas, and the Latics had more shots, corners, and possession in the match. City also committed more than twice as many fouls as Wigan (17 v. 8). But is Martinez right? Can teams gain points when they foul? What, if anything, is the correlation between fouls and performance indicators? To answer the question with some degree of reliability, I went back to the last five seasons of the Premier League and examined individual match data. In particular, I calculated the correlation between the number of fouls teams committed and the number of points they won. Here's what you see when you graph that statistical relationship for the league as a whole for each of the last five seasons. The yellow line shows the best fit to the data; the grey shaded area tells us that 95 out of 100 times, the correlation will fall into the range shown.
Looking at the facts of the match, it's hard to quibble with Martinez's assessment. For much of the match Wigan were the only side with any ideas, and the Latics had more shots, corners, and possession in the match. City also committed more than twice as many fouls as Wigan (17 v. 8). But is Martinez right? Can teams gain points when they foul? What, if anything, is the correlation between fouls and performance indicators? To answer the question with some degree of reliability, I went back to the last five seasons of the Premier League and examined individual match data. In particular, I calculated the correlation between the number of fouls teams committed and the number of points they won. Here's what you see when you graph that statistical relationship for the league as a whole for each of the last five seasons. The yellow line shows the best fit to the data; the grey shaded area tells us that 95 out of 100 times, the correlation will fall into the range shown.
The graph shows a couple of interesting patterns. First, in each of the five seasons, more fouls are associated with fewer points won. But which way does the causal arrow run? Do bad teams foul more or does fouling more lead to fewer points?
The relationships between fouls and points or fouls committed and suffered are undoubtedly complex. From earlier analyses we know that less successful teams on average commit slightly more fouls. At the same time, bad teams don't always foul more; more importantly, while less successful teams may foul slightly more, this doesn't mean that more successful teams are fouled more. In fact, Martinez would argue it was just the opposite for his team playing Manchester City.
So what we can say is that there clearly is a correlation, and it's negative for the league as a whole.* So teams that foul more win fewer points and teams that win fewer points foul more. Fouling seems a costly strategy, especially considering that fouls tend to beget fouls from the other side.
The second and perhaps more interesting pattern in the graphs is that the connection between fouls committed and points earned looks very similar for four of the five seasons. From 2006/2007 to 2009/10, the statistical relationships look very much alike. But the slope of the line is significantly shallower for the 2010/11 season. The obvious question is: why?
Turns out, fouls mattered to the the fortunes of different clubs in different ways. In category one, we find several clubs that were significantly more likely to lose points in matches when they fouled a lot. These included some of the very best clubs, including Arsenal, Chelsea, Manchester United, and Liverpool, but also Sunderland, and West Ham. Then there was a group of teams to whom it made no difference whatsoever. These included many of the mid-table clubs (Birmingham, Blackburn, Bolton, Fulham, Stoke, Tottenham, or Wigan). And then - most intriguingly - there were clubs that not only seemed to get away with rough play but actually prospered when they fouled more: teams for whom fouling seemed to pay off included Newcastle, West Brom, Wolves, and yes, Manchester City. I presume that's some amount of vindication for the Wigan manager - but hopefully also food for thought as the Latics' fortunes were insensitive whether they played rough or gentle football.
* In case you're wondering, it also doesn't matter if we measure fouls by the total number, the ratio of fouls (Team A/Team B), or the absolute difference in the number of fouls in a match. The conclusion about a negative correlation between fouls and points for the league as a whole remain the same.
