Why Chelsea score so few goals and how crucial big chances are
Chelsea has scored just six league goals since November 6, 2022 (12 league games). The question ‘how crucial are big chances?’ is very much tied into the expected goals (xG) debate and how reliable it is as a model. Many claim most xG models are incomplete. The fact remains that many users of xG don’t know how to use it, and keep trying to use it for what it wasn’t meant for. xG can be measured for players, but can also be measured for teams, and today we’d be focusing on the latter.
The reason xG is important is because big chances are defined by xG value, amongst other things. Opta defines it best, but a big chance is a chance where a player is favoured to score with respect to the goalkeeper. The big chance definition takes into consideration the position the shot is taken from, the path of the ball to the goal and the amount of pressure on the player shooting the ball. Therefore, a big chance has a minimum xG value of 0.25.
This means the average player is expected to score a big chance 25 per cent of the time. The player in “average player” refers to any player, not just forwards or strikers. A good big chance conversion rate for a striker would be 40-45%, the best finishers hover around the 47-55% range over a full season when you consider shots taken and goals scored. Anything above 55% is outrageous.
It should be mentioned that the more big chances a player gets, the more likely their conversion rate would reduce over the course of a season. We now know what a big chance is, and what big chance conversion should look like for players. But how should they look like for teams? Why are they important? Why should teams strife to create as much as they can? Shouldn’t they just focus on getting goals wherever they can, regardless of whether it’s from a big chance or not? Should players not just be better at shooting the ball when their teams aren’t creating many big chances? Should teams not just get better shooters when they’re struggling to score?