Paul Riley (among others) has done some great work looking at Manchester United's shooting habits over the last couple of seasons. We thought we would analyse the data to take a closer look at this phenomenon.
To recap, Manchester United have scored lots of goals this season, far more than everyone else. However, despite this they have taken relatively few shots in comparison to other teams in this season's Barclays Premier League. The graph below demonstrates this - six teams have taken more shots than them (note, Chelsea are hidden under Arsenal on this graph):
They are scoring with nearly one in every five (non-blocked) shots they take (19.0%), compared to a league average of 13.2%. Even Chelsea, the side with the second best conversion rate are only scoring with 16.4% of shots.
As a stand-alone metric, shot conversion rate is, as a rule, not especially predictive from year-to-year. Below is a scatter plot of teams' conversion rates in successive seasons over the last 10 years of the Premier League:
While there is a clear positive correlation, it's not all that strong. Therefore, on the face of it we might expect United's conversion rate to regress significantly in 2013-14. Turns out though, they've been doing it for a little while now. The orange dot in the top right corner is United's 11/12 & 12/13 seasons, while the blue dot immediately to the left is their 10/11 & 11/12 seasons.
Indeed, those three represent three of the top 10 conversion rates in the last 10 Premier League seasons:
That their conversion rate is only being discussed recently despite being effectively unchanged since last season is likely due to the different narrative of the two seasons. Last year, City were also dramatically 'outperforming' in terms of conversion rate, making United's performance apparently less notable:
So it's clear that this has been a considerable factor in the team's performance for a while.
However, this still doesn't answer the question of how United are maintaining these high conversion rates - which have been climbing since the 08/09 season (see below), or if they are indeed sustainable.
Over the last seven years, their number of shots per game has been decreasing but their average chance quality (excluding penalties) has been going up at the same time. So, it would appear they have been trading a volume of shots for ones in better positions.
This can partly be attributed to Cristiano Ronaldo's departure to Madrid - during his time in Manchester he took a comparatively much larger number of shots, but from relatively inferior positions. The graph above reflects this - post-2009, there is a definite drop in the number of shots taken, and a gradual improvement on shot location.
In order to demonstrate this further, we can see this in the average location of their players' (non-dead ball) shots over those seasons:
Here, colour represents the average chance quality (red is higher, blue lower), with the size of the point the number of shots attempted.
If we revisit the table from earlier and add in the average chance quality, we can see United's current and previous seasons rank as the two highest in the seven seasons for which we have the data.
Almost all of the other teams have/had relatively good quality chances, with the exception of Chelsea this season who appear to be converting their chances at a very high rate in spite of fairly average chances created.
As Paul Riley says in his article, "United have been relentlessly good at converting these shots into goals in the central area of the box". As defined in this article, our shooting model backs this up - United's average chance quality over the past two seasons is the highest on record.
Of course, while United are creating very good chances, that is only one part of the equation (just ask Liverpool fans last season). They are however putting them away at a superb rate as well - still exceeding their expected tally based on chance quality by a league-best 10 goals thus far.
This is no mere van Persie effect either (eighth this season at +3.5 goals), as they are getting contributions from throughout the team - with six players ranking in the top 30 in the Premier League for goals above expectation:
The analysis of Manchester United's runaway success this season has pointed at least one finger at their apparently unsustainably high scoring rates. This is not a one-season effect however, with United only marginally exceeding their conversion rate from last season. Over that time, Ferguson's side have been creating extremely high quality chances and, this season in particular converting them at a very good clip. While the influence of Rafael and Evra on the latter suggests the effect may not be wholly sustainable, until opposition teams manage to work out a way to stop United getting into these positions their goal-scoring efficiency could very well continue.