Save % - A Reminder that Goal Keepers are High Risk

Twitter was buzzing with Arsenal signing Petr Cech, for £11m. An amazing sum for a 33year old GK. Plenty of work has been done by others in this field showing that their is next to nothing repeatable about the GK position. This lack of ability to narrow down any sort of skill, should make clubs afraid to spend, even a small amount of money, on this position.

Many have praised Arsenal for getting a proven and experienced GK who makes Arsenal title contenders:

.@ChelseaFC's Cech could turn @Arsenal into genuine title contenders, writes @nicholaspwright http://t.co/wUbRSBpo4f pic.twitter.com/toT17QhJYg

— Sky Sports Football (@SkyFootball) June 22, 2015

Then there are others like @footballfactman, (probably the best GK analyst in the blogosphere)
who is less excited:

@BobbyGardiner he's excellent in right set up. He was dire when AVBs high line exposed him pic.twitter.com/xrBjdQgVlO

— Paul Riley (@footballfactman) June 22, 2015

My point here is not about Cech but GK's in general. They are voodoo.

I have ran a few scatter plots to show the linear regression of team save% over various lengths of time and none show any signs that a team or keeper is worth 'betting' on both in the short term or long term, at least when looking at saves per shot on target against (save%).

These results will be nothing new to most of you but reiterates that teams and GK's lack a lot of control over goals scored against them. Analytically, it is pretty tough to endorse keeper x, y or z. What we see one year or even for 2 yrs in a row (unlikely) can change very quickly.

Arsenal are not a poor club and can absorb this move without missing much of a beat, financially, but based on linear regression the best we can predict is that Arsenal will have a save% close to league average, in the 2015-16 season (like every other team).

Cheers,
Clarke
Follow @footyinthecloud