Initially, in the days well before squad numbers, 12 was universally the number of the sub. A player whose versatility was often prized above his raw technical talent because a side was allowed but a single roll of the dice. Tactical alterations, by necessity, took a back seat to the threat of enforced replacement because of injury. Mick Bates was a prime example at Leeds, for those who remember the 60's and 70's.
Nowadays substitutions are much more about tactical change or the virtually premeditated replacing of tired legs or brains with equally talented players from a squad system that has grown well beyond the constraints of a 1 to 11 numbering system for the starters and 12 for the single, track suited replacement. A lack of innovation that possibly denied Jimmy Greaves, suited and football booted involvement in a Wembley World Cup Final, has evolved from it's sticking plaster beginnings to become an essential, present day part of managerial match day planning.
However, more fundamental information can be mined relatively easily, most notably if substitutions have any age related bias. I've previously looked at age effects on such easily measured attributes, such as goal keeping skill and age decayed playing time and scoring patterns, so I'll use a similar approach here.
As anyone knows who has seen her number lifted aloft by the fourth official, there are two sides to every substitution and the format of much of the data readily allows for evaluating age profiles for the replaced players rather than those who are subbed onto the pitch. The age profile of the starting 11 is readily obtainable. Therefore, we can know with accuracy the make up of the pool of players from which the manager is choosing his replacee. By contrast, profiling the replacement requires accurate knowledge of the make up of the entire and always partially unused bench and that is much less common information.
Footballers appear to reach a peak of performance in their late twenties and so it is not surprising to see that this age group has disproportionately more players active in Premiership sides. Extremes of youth and age, while present are much less numerous. Therefore, to try to add context to any age related substitution strategy common within the EPL, we need to know the relative proportions of each age bracket within a starting eleven across a reasonable sample of matches.
Once we move onto see how frequently a particular age of player is substituted, more problems arise.
Firstly, some substitutions are forced, rather than tactical.
Secondly, a player subbed for the final minute to receive a standing ovation for an excellent performance or in the case of Jamie Carragher in May, an outstanding career, is different to a replacement on the hour, possibly because of poor form or an anticipated improved contribution from the newly introduced colleague.
Thirdly, keepers, not only have slightly atypical aged related performance profiles, they are also very rarely replaced for reasons other than injury.
To combat these issues, we can eliminate keepers from the data, hope that injury replacement is largely dwarfed by tactical, manager inspired change and vanity subs can be allowed for by using the proportion of time absent from the field in the case of the subbed individual, rather than merely counting every subbed out player incident as being equal.
We are now in a position to try to see if some age groups are subbed out of a game proportionally more or less often than you may expect compared to their initial representation amongst the starting eleven.
Unfortunately, the plot is cluttered to try to maintain information, but essentially the paired red and green columns show the proportion of players of one age group who were present in the starting eleven and thus available to be subbed out of the game (red column) and the rate at which they actually suffered substitution, measured as minutes absent from play as a proportion of all playing minutes lost by replaced players (green column).
For example, 24 year old players comprised 6.8% of starters, but accounted for 8.6% of the time spent sitting on the bench as a replaced player. By contrast 29 year olds were 9.5% of the starting population, but suffered just 7.3% of the substitutions.
The salient point is depicted by the orange and black arrows. The black arrow shows the age groups that were replaced more frequently than would have occurred from a purely random selection process and the orange arrows show the groups that were allowed by the manager to stay on the pitch more frequently and possibly for longer on the occasions when they were replaced.
To cut through the detailed definitions, younger players tend to get subbed out of a match more frequently than older ones, once you account for the relative frequency of each age bracket.
These samples come from all EPL sides for a significant portion of 2012/13, if they survive the necessary assumptions and are repeated in larger samples they offer an insight into the choices being made on the bench during a game. The mangers appear to be making a conscious decision that balances youthful energy, but possible inexperience and lack of peak technical ability against possibly lower fitness levels, but much more experience and appear to come down in favour of removing youth (even if they are replacing it with more youth) and keeping with experience as the game enters it's last, tactically active final half hour.
Players that are still playing past the general ageing peak are a biased selection of the very best, so it is likely that their contribution in a sport of technical, cerebral and physical demands will overall outweigh that of younger, and not yet fully realised talent.
Alternatively, the plot may do no more than demonstrate a safety first attitude that placates senior pros and fans alike at the expense of a potentially more profitable and adventurous selection policy.