How Well Do Individual Scoring Rates Survive A Change Of Scenery ?

The opening of the transfer window always brings with it a huge amount of speculation concerning the possible destination of unsettled, out of favour or much sought after players. Many of these apparently "done deals" fail to materialize and the only real certainty surrounding the January spending spree is that the window will "slam shut" at the end of the final day.

Previously much of the pairing of players to their ideal destination was done on gut feeling, but the ever increasing amount of individual data  has now enabled even the casual fan to pick out that tough tackling midfield enforcer who will propel his side to either safety or greater heights. So has the readily available shooting, scoring and tackling statistics made completing the jigsaw an altogether simpler process ?

Goals are widely used to define strikers and in this post on super subs, I used the number of goals scored by Dzeko for Manchester City as a proportion of all goals scored whilst he was on the pitch in an attempt to quantify his contribution in both roles. This method is attractive because it partly accounts for quality of opportunity, a rout against a poor side will see other players also getting on the score sheet and it also allows for the heightened rate of scoring later in matches. Also a player scoring two goals in a first half isn't penalized if he is absent for the second period and further goals are added to the total.

By normalizing the scoring environment, while Dzeko's team mates remain reasonably constant, we can try to judge if there is likely to be any difference between Dzeko, the starter and Dzeko, the sub. We can further develop this approach to try to estimate how a newly acquired striker may fit into a new team, especially as measured by the bottom line of goals scored.

Estimating player ability is always difficult, ageing and survivor bias for example is hardly ever addressed, but the most glaring problem in this instance is that transfer targets are playing for a different team, alongside players of differing abilities and varied tactical priorities compared to the side which is pursuing him. A player may score a high proportion of the goals claimed by a struggling Premiership side, but that may be because he is head and shoulders above his striking team mates. The numbers may be telling you that he is "too good" for his present side, but is he good enough to play for a potential suitor ? A potential buyer needs to know how he is likely to perform for them and raw scoring exploits are unlikely to provide reliable conclusions. Are you buying a squad player or an upgrade ?

The case of Daniel Sturridge highlights how we may be able to make more informed predictions using the  scoring exploits a player achieves whilst playing at different clubs. Sturridge has performed in the top flight at the then mid table Manchester City, Championship winning Chelsea, struggling Bolton, by virtue of the loan system and now appears to have found a level where appearances and the opportunity to showcase his talent will be guaranteed at Liverpool.

During his three seasons at City, he scored 30% of his side's goals whist he was on the pitch. At Chelsea the figure dropped to just below 20% and in a half season loan at Bolton it shot up to nearly 50%. Sturridge's opportunities have been limited, so sample sizes are small, but his "talent" as a striker appears to rise as the overall quality of his side falls. His proportional strike rate was exceptional at Bolton, very good at City and good at Chelsea.

The reality is probably that he was close to being the same player at all three clubs, especially during his time at Chelsea and Bolton. He shone outstandingly at The Reebok because he was the best attacking player on the team and as such was able to dominate his fellow strikers. Once he arrived back at Chelsea he was partly eclipsed by better strikers and his proportional rate of goals scored fell. Had Chelsea purchased, instead of merely recalling Sturridge from Bolton on the basis of his 50% strike rate, they would have been disappointed if they had expected that headline stat to be repeated at The Bridge.

In the table above, I've added the proportion of goals scored by a variety of strikers who have played for multiple seasons at Premiership clubs of differing overall quality. The strikers included Crouch, Adebayor and Bent in addition to Sturridge and the clubs involved range from Chelsea and Arsenal, down to struggling EPL sides like Southampton and Charlton. The relationship which appears to exist for Sturridge also shows up in this larger group of similarly talented strikers, each of whom have been traded for eight figure transfer fees over the course of the sample. They tend to score a smaller proportion of the goals scored as they move to bigger and better clubs.

When playing for struggling EPL sides, denoted by success rates in the region of 0.35, the line of best fit indicates that such strikers are likely to account for between 40 and 50% of the team goals scored whilst they are on the pitch. When making the step up to a Champions League quality side, their contribution then typically falls to around 20% because they are probably surrounded by a bigger pool of goal scoring talent.

Crouch, Scorer of 22% of Liverpool's Goals when playing compared to 35% for Stoke.

Collecting the proportional scoring records of players provides an added level of information which may prove valuable in predicting future performance at other clubs. Knowledge of a player's record at his previous clubs may enable a side looking to buy in the transfer market to estimate the proportion of goals a new signing would typically contribute at a different class of club. And more importantly whether this figure will go hand in hand with an overall increase in total goals for the team. Sturridge's presence in the Bolton lineup also coincided with a 10% increase in total goals scored compare to their previous 30 games, although this needs to be confirmed in much larger sample sizes.

Demba Ba's record at West Ham and Newcastle has seen him score around 40% of his side's goals as an active player at teams with a combined success rate in the region of 0.44, well in line with expectations. If he continues to hug the regression line, Chelsea may have purchased a player capable scoring 15% of the goals at a club such as Chelsea. Further research may show whether this will be an upgrade at the London club, although the reduced price tag for such an apparent talent would already appear to be good value.

Luis Suarez, Daniel Sturridge's intended strike partner at Liverpool also fits neatly onto the line of best fit. He has scored around 30% of the goals when he's played at Anfield, just the figure you would expect for a top striker playing for a team with an overall success rate of 0.54 spread over two part and one completed season. The figures for Sturridge and Suarez therefore point to Liverpool having now acquired two top class strikers. The hope will be that the pair will produced strike rates nearer to 20%, provide scoring opportunities for other team mates through assists or by occupying opposition defences and Liverpool's success rate as measured in wins and draws will increase in line with the profile of their new striking threat.

Liverpool's previous big money swap saw Andy Carroll join from Newcastle. Carroll and Newcastle spent 2009/10 in The Championship, but during his time playing in the Premiership, Newcastle's success rate hovered just below 0.4 and a top striking prospect playing for such a struggling side should have scored around 45% of the team's goal. Carroll only accounted for 35%, so on that basis he was a much bigger gamble than the one they've taken on Sturridge, who cost less and has better numbers.

This methodology is reasonably straightforward when used for goals, but it is equally feasible to apply it to add context to other on field actions that could otherwise potentially lead to misleading conclusions. Looking at the number or even the proportion of successful passes or assists may be just the start of the selection process and not the endgame.

Also check out Danny Pugsley who takes at look at the subject here