Tuesday 27 June 2023

The Ageing of the Ageing Curve

A long, long, long, long time ago (2013) there wasn't that much granular data around and certainly hardly any based around metrics that eventually led to xG entering the mainstream of football/soccer analytics. Therefore, proxies abounded and share of playing time to evaluate a player's ageing rise and fall quickly became a go to method. Pull enough player data for minutes played, trawl through wiki for dob's and you came up with a pleasing curve that rose from the teenage years to the early twenties, peaked in the mid to late 20's and fell away as the ex pro went away to impart gnarled cliches on Sky. Plot the season by season change in playing time, the delta method & the linear trendline cut the axis where it was assumed that effortless push came to more laboured shove.
When a player's minutes finally fell instead of maintaining an ever shallower upward trend, the assumption was that he had become less effective on the field, performance levels had fallen, the manager had taken note and action had ensued. Physical decline, it was assumed, had begun to outstrip experience and smarts. We're now over a decade further down the road to analytical enlightenment, where on and off ball stuff gets routinely measured, even if there's still no consistency in naming metrics. Creativity, shooting execution & positional sense, ball progression via passes or carries & risk/reward has become less blurry & more transparent. We've also seen advances in sports science to prolong a the peak age of performance & witnessed anecdotal evidence for increased player longevity, even in demanding roles. So it's well overdue to usher "share of minutes played" into the lobby, treat it as just a fraction of what happens along the age curve & try to understand what might be going on in a player's career arc. Non shot expected goals added was one powerful metric that sought to measure by how much individual ball progression improved a team's likelihood of scoring. The delta approach to non shot xG added per 90 shows a gradual increase in performancein the early years of a player's career, but thereafter there's virtually no change, on average in the performance levels achieved per 90 minutes. It's not quite an expected trendline, more early improvement, but then flatlining.
The ball progression illustrated combines passes & carries and it's generally accepted that the latter is more physically demanding than the former. Perhaps players are replacing any shortfall of xG added via carries by upping their output from passes. To see what may be happening I looked at how NS xG added from just carries has changed over the last three completed seasons for all players as they age and again there's virtually no change on average in the rate of xG added from carries as a player ticks off their birthdays.
Persuasive speculation that as a player ages, their performance levels dip & they get left out of the side more frequently, kept "share of minutes played" a respected metric for nearly a decade. But how the new quantifiable metrics don't change that much well into a player's 30's may suggest that there is a slightly different dynamic at play for individuals, overall. Namely, player metrics, even more physically demanding ones which involve ball carrying, can with good managing of playing time enable players past what was considered their prime to maintain their own high standards. In short, you might get very similar levels of performance in a player's early 30's as you got in their mid 20's.....just not quite as often as you did previously.