Unlocking the Key Pass

A couple of weeks back, we looked at the correlation between shots on goal and goals scored and (unsurprisingly) found a pretty strong link between the two. This ground breaking research probably won't make it into the next Freakonomics book but it might help illustrate players who are getting good looks at goal but not converting their chances. I went on to mess up my explanation of regression to the mean (h/t to mustardking for correcting me) but in essence, while players who have yet to capitalise on a number of shots on target, don't suddenly find themselves 'due' a goal, if a player continues to get in good positions and take good shots, over a period of time we would expect them to score at their historical rate going forward (normally around a goal every three shots on target for elite players).

Building on the shots analysis, we can now move onto to trying to predict assists. This is inherently trickier as the inputs we need to use will almost certainly involve some kind of judgement. 'Shots on goal' is a pretty unarguable number (I saw 'pretty' because there could be some issues with deflected shots, tame shots etc) and the number is provided by a number of sources. For assists though, we need to look at things like 'key passes', 'successful crosses' and 'through balls' which could be interpreted differently by two different people watching the same game. Nevertheless, we deal with what we have, and I will be using Opta data for all the below analysis, except where otherwise noted.

Trend Spotting
Opta provides a number of passing stats which can look at for signs of correlation with assists. Using data from the 2010/11 season, we get the below correlations:

• Total passes made 45%
• Successful dribbles 49%
• Successful passes in opponents half 53%
• Successful cross 69%
• Total crosses 70%
• Key passes made 73%

The first three are clearly, and unsurprisingly, not too useful and with the lack of distinction between successful and total crosses am I going to categorise these as a single item. That leaves us with crosses and key passes made, each of which are solid indicators of assist success. For now I am going to focus on key passes as this metric will include all player types, rather than leaning towards wide players or players from teams who play a certain formation or style.

Defining Average

Over the course of the season we are going to identify players whose key passes are not being converted at an 'average' rate and thus should see some improvement in the future. For example, let's imagine that after 15 gameweeks Chris Brunt has played 30 key passes but only has 1 assist. At that point he wouldn't be having a great fantasy season and would likely be hovering below the radar. However, the low rate of conversion is probably unsustainable, and while we we can't say that future chances will be converted at a greater rate to 'equal everything out', we can expect future chances to be converted at an average rate, and thus Brunt would be more valuable than he had been to date. But what exactly is the 'average rate'? 

Last year, Opta tells us that there were 4,941 'key passes' made by midfielders and forwards with 516 of these converted into goals (thus generating an assist). That gives a league wide conversion rate of 10.4%. This is a good start, but not all teams are equal. Slipping a ball through to Wayne Rooney and Javier Hernandez (who converted 121 shots in 24 goals) is very different to passing to Huge Rodallega and Franco Di Santo (who converted 122 shots into 10 goals) and some account needs to be taken for this. The conversion rate of 'key passes' to assists for each team last season were as below:

1. Man Utd 16.4%
2. Arsenal 13.1%
3. Man City 12.3%
4. Blackburn 12.0%
5. Everton 11.4%
6. West Brom 11.4%
7. Wolves 11.2%
8. Bolton 11.0%
9. Newcastle 10.9%
League Average 10.4%
10. Fulham 10.2%
11. Stoke 9.8%
12. Aston Villa 9.2%
13. Liverpool 8.6%
14. Sunderland 8.4%
15. Tottenham 8.1%
16. Chelsea 7.9%
17. Wigan 6.5%

While these numbers clearly vary considerably, we must remember that we are dealing with small sample sizes and not overreact. Only the top and bottom two teams lie more than a single standard deviation away from the mean so I think we should be fairly comfortable using the ~10% conversion rate as a league wide benchmark (taking care to note that, generally, good teams will have higher rates).

The Season So Far

It's obviously early to start making assumptions, but let's take a quick look at who has enjoyed the most key passes to date, and how this has translated to assists. Where this isn't the case, it's likely that these players are currently being undervalued and might be decent differentiators for the coming weeks:

Player	Key Passes	Assists	Conversion Rate
Lampard	15	2	13.3%
Murphy	14	2	14.3%
Silva	11	2	18.2%
M Petrov	11	2	18.2%
Nasri	11	4	36.4%
Nani	11	2	18.2%
Watson	10	0	0%
Faurlin	10	0	0%
Ramsey	9	0	0%
S Petrov	8	0	0%
J Morrison	8	0	0%
Larsson	8	0	0%
Duff	8	0	0%
Arshavin	8	0	0%
Agustien	8	0	0%
Taarabt	7	0	0%
Sessegnon	7	0	0%
Arteta	7	0	0%
A Young	6	4	66.7%
Mata	5	0	0%
Wright Phillips	4	0	0%
Milner	4	2	50.0%
Meireles	4	2	50.0%
Malouda	4	0	0%

This is not an exhaustive list of course but I have tried to highlight those players with interesting numbers to date:

• It is interesting to note that the league wide conversion rate for the season to date has been 10.7%; very close to the comparable number last season (10.4%)
• This doesn't categorically prove anything, but it is another piece of evidence suggesting that Silva may still be the choice pick over Nasri. Nasri has more assists to date but such a high conversion rate looks unlikely to continue all year. You can argue that maybe he plays better quality 'key passes' than Silva but playing the numbers alone, their value looks very close with regards to assists but Silva enjoys a goal scoring advantage.
• Those players from Watson down to Arteta have been playing dangerous balls but have yet to have one of their gems converted by a teammate. Considering that we would have only expected a single assist, let's not get carried away and suggest they will enjoy huge seasons from here on,. That said, this might serve as a useful tie breaker for a player like Ben Watson who has also assumed penalty duties and logged a few shots on goal already this season.
• Ashley Young has been great since signing for the Champions but that 67% conversion rate is unsustainable and unless he starts delivering more key passes, we would expect his assists to slow down a tad over the coming weeks as the conversion rate normalises.
• I've been pretty down on Chelsea over these opening weeks but this data suggests Lampard will continue to generate assists, which coupled with his penalty duties, might just give him some value this year. I added Mata to this list just because his 5 key passes have been generated in just 96 minutes of playing time. If he keeps averaging close to 5 key passes a game, the assists will surely start to come his way very soon. Finally for Chelsea, Malouda led the league by some distance last season with 113 key passes (Henderson and Downing were second with 78) but he's managed just 4 in 217 minutes this year.
• Shaun Wright Phillips looked very dangerous tonight for QPR and this is reflected in these numbers with 4 key passes in just 88 minutes. At 6.4m he is too expensive to be a bench player so we probably need to see some QPR goals before we buy in, but the potential for a return to prominence is there (playing alongside Barton and Taarabt, who was excellent in the first half, will also help).

I will continue to track these numbers as the season progresses and will highlight players with high 'key pass' numbers given the strong correlation with assists. Continue to check back this week or follow on Twitter @plfantasy as I plan to post this week on how to deal with the two Manchester teams along with the weekly look at the chalkboards. Thanks for reading and please post comments/suggestions/questions below or over on Twitter.