Now that the transfer window is open, lots of managers are looking to strengthen their squads. Managers like Everton's David Moyes seem typical; he's looking to sign a striker, having repeatedly bemoaned his team's lack of ability to finish in front of goal, most recently after the away draw at West Ham. Alex McLeish, too, is looking to strengthen Birmingham City's front line to convert good offensive play into actual goals scored. Even Carlo Ancelotti is talking about needing some help up front for Chelsea, where Nicolas Anelka has been having a slow season and Didier Drogba is not in great form.
One way for the numerically-minded fan to see if these managers are correct in assessing their team's weakness as the lack of finishing is to take a look at a team's conversion rate for the first half of the season. Conversion rates are conceptualized as a team's ability to turn accurate shots into goals; it is measured as the ratio of goals scored to shots on target. I think this is a particularly useful measure of offensive production because it goes beyond simply counting how many times a team got anywhere near the other team's goal or just got a shot off - many of these incidents never pose a threat to the other team. Instead, this measure indicates how threatening shots on goal were - were they mostly weak and badly aimed balls, or was there more to them? To help with this, I have calculated teams' conversion rates as a key metric of a team's offensive production. Take a look.
The average conversion rate in the EPL this season is .227 (or about in 1 goal in 4.5 accurate shots). But clearly, there is also wide variation in conversion rates, as I have reported in previous posts. This season is no exception. Conversion rates range from a low of slightly above .1 (1 goal per 10 shots on target) for Wigan to slightly over .3 (1 in 3) for Blackburn and Newcastle. These latter teams have done exceptionally well once they find themselves in front of goal and are able to get an accurate shot off. The Top 5 teams in conversion so far this season are (in order): Blackburn, Newcastle, Sunderland, West Brom, and Man U and Stoke. The Bottom 5 are Wigan, Fulham, Everton, Aston Villa, and West Ham. So, based on these numbers, Moyes is right to look for reinforcements, as would be the managers of some of these other teams. In contrast, Birmingham's .25 conversion rate is actually slightly above average. So for Birmingham, the problem there does not seem to be conversion but getting the ball in front of goal. Evidence for this is that Birmingham's average number of shots taken per match (including inaccurate ones) is 9 so far this season, while the EPL average is 12.4.
So does it matter to teams' conversion rates if they are playing at home or away? The answer to this question is slightly surprising. It actually does, but in unexpected ways (at least to me). Take a look at the following graph, which lists conversion rates per match for home and away matches.
While average conversion rates are essentially identical at home and away (.230 v. 224, respectively), the data show that there is greater variation in conversion rates when teams teams play away from home than when they play at home. That is, the lowest and highest conversion rates can be found in the away panel of the graph. Aston Villa and West Ham's woeful away performance of .099 are the lowest conversion rates in the Premier League this season, but West Brom's .38 conversion rate, achieved when playing away from home is the best in the Premier League this season. Part of this has to do with the fact that West Brom have scored about as many goals at home as they have away (1.375 v. 1.444 - slightly more on the road, actually), but they have needed many more shots on goal to get there at home than on the road (15.1 v. 10.4). And Everton's woes are more pronounced at home than on the road, interestingly, while the teams that need the most help in this department clearly include Wigan, and Fulham.
More to come on defensive production soon. Enjoy the second half of the season.
