Thursday 30 August 2012

A Keeper's Short Passing Ability. An Insignificant Stat That Tells You So Much.

One of the challenges of the recently released Premiership data is trying to marry the figures to on field incidents while at the same time avoiding drawing misleading conclusions because of lack of context or lack of extra detail. Passes and pass completions are one of the most comprehensively covered areas of the data set released by MCFC. Individual passing events are broken down by length, general pitch position and direction, enabling anyone to build  an impressive database relatively quickly.

Below I've summarised the completion rates for all short passes attempted by players during the last campaign. As you would expect, passing is a core footballing skill and the majority of players excel at it. Overall completion rates for all players attempting short passes in all areas of the field exceed 80%

Completion Rates For Short Passes In The Premiership, 2011/12 Season.

Player Position. Completion Rate % .
Striker. 75.8
Midfielder. 84.7
Defender. 83.9
Keeper. 92.1
All Players. 83.3

However, once we break the completion rates down by position we immediately notice a problem. Seemingly, the most accurate practitioners of the short pass as a group are goal keepers and the most accurate short passer of a football in the EPL last term was MCFC's Joe Hart. With all due respect to Joe, when data throws up broad results that are so obviously at odds with reality, then the analysis of the figures is plainly flawed.

Thomas Sorensen....Stoke's most accurate short passer last term ?

In this case the cause of the anomaly is extremely easy to spot. If a keeper misplaces a short pass he may quickly go from being the last line of defence to the only line of defence. Therefore, keepers are only likely to attempt short passes if they are extremely confident that the pass will be completed. Short passes rolled out to an unmarked defensive colleague are much easier to complete than an attempt to thread a 3 yard pass to a fellow striker in a crowded area.

So a goalkeeper's superior passing completion rate comes about because of the type of short passes they choose to attempt. We can further confirm our suspicions that keepers are making predominately easy short passes by looking at the spread of short passing talent that appears to exist between last year's batch of Premiership keepers.

The easier a task is to complete then the more difficult it becomes to quantify the different levels of skill of those undertaking the task. For example, asking a group of individuals to work out 2+2 won't really help you if you are trying to sort the mathematically gifted from the merely numerate. If we calculate the spread of short passing talent within 2011/12 keepers, we discover that the apparent difference in talent is indeed tighter for keepers compared to the values we see for defenders, midfielders and strikers. In short the keepers are, probably wisely, asking themselves easy questions.

A final additional conclusion can be drawn from this trawl through the passing ability of goalies. After correcting for such variables as number of short passes attempted, we find that teammates who shared duties last year appear to have similar levels of passing ability. Sorensen and Begovic, Jaaskelainen and Bogdan, De Gea and Lindergaard, Given and Guzan, Cerny and Bunn, Reina and Doni, each pair's true short passing ability is within just a few percentage points of each other. This may be coincidental, but it's more likely to also reflect the ability of the relatively constant group of players to whom they are passing. A pass is an interaction between two players and if the receiver is constantly allowing the pass to roll under his boot, his mistake will be reflected in the conversion rates of the passer.

A keeper's passing ability isn't near the top of a scout's must have list, so listing their regressed conversion rates for each keeper based on last year's data isn't likely to interest too many people. However, in piecing together the reason's behind Joe Hart's apparent excellence we have uncovered factors that almost certainly contribute to pass conversion rates for other players in other positions within the team.

Explicit mention of the difficulty of the pass choice is absent from the aggregated list, but it appears to be such a large contributor to conversion rates that it can even elevate goalkeepers as a group ahead of presumable more gifted passing midfielders. It is therefore a hugely important factor. A keeper's choice of the easy short passing option compared to his outfield teammates was easy to spot, but less noticeable is discrepancy in pass difficulty choices made by players who occupy similar positions in a team. The player with the higher completion rate may also be choosing an easier option and that has implications for player evaluation and balancing the risk reward of on field actions.

By looking at an obscure goalkeeping statistic, we've highlighted the need to try to quantify the levels of difficulty involved in individual player's on field attempts, the need to acknowledge the level of talent surrounding him and to analyse using only data derived from players plying their craft in similar playing positions, where the risk reward balance for passes will be similar.

The next post will look at the how we can begin to address these issues and contextualize passing for outfield players using the available MCFC data.

No comments:

Post a Comment