Alright, so caveat up front: there is no magic number that tells you how good a goalkeeper (or any position for that matter) is. And I am not an expert in this, just someone who finds it interesting as another layer to look at in addition to actually watching the player play. I know plenty don't, including the poster above
But you asked, so here we are
Particularly in this case, there's a few categories that give a bit of a picture as to what kind of a keeper Ramsdale
was last season. They don't tell us what he'll be like next season (probably). I have no doubt he'll get better given that he's young, that was his first season as a number 1 at this level, and I'd be willing to bet our goalkeeper coaching setup is better than Bournemouth's.
From
FBRef, a measure of shot-stopping ability compared to the rest of the league is
PSxG+/- (what a mouthful). If you mouse over the heading for the stats it shows you an explanation for each one - this one is "how likely a goalkeeper was to save a shot [based on data, not opinion]", minus the actual number of goals they conceded.
Dubravka absolutely smashed this with a +10 measure which basically he means his saves were "worth" 10 goals above the average: he faced a PSxG of 66 goals, and conceded 56 (note - he actually conceded 58, but 2 were own goals). Henderson was +4.6 - his saves were worth about 4 or 5 goals over the full season. Ramsdale was +0.1 - he did exactly what was expected of him, essentially. On the flipside, Kepa actively harmed his team this season at -9.8.
To an extent this can get skewed by how much work a keeper has to do. If a keeper doesn't face a single shot on target, he'll struggle to get that number up to Dubravka levels, because he literally has no saves to make. So Dubravka and Guaita being top of this might just mean they were only quite good, but very busy. But as the explainer on the site says, these numbers "suggest better luck or an above average ability to save shots". And it is over the course of a full season, so a decent sample size.
There's a good article on it here from Statsbomb (who provide the data that FBRef use):
https://statsbomb.com/2018/11/a-new-way-to-measure-keepers-shot-stopping-post-shot-expected-goals/
Also worth looking at things like Save %, Crosses Stopped %, and #OPA (number of defensive actions outside penalty area - how often was he out sweeping up) just to give you a bit more of a picture of how he fared last season compared to all the other keepers in the league.
