Expected Goals suggest which teams have been: the good, the bad, & the lucky

James Tavernier

written by – Rangers Report

One of the advanced stats from the hockey world that has gained traction in football has been PDO – which is basically a measure of which teams are succeeding or struggling based on fortunate or unfortunate results.  Basically, which teams have been lucky & which have been snake bitten.

So what is PDO?  Clare Austin explains, in her hockey blog Puckology“What PDO does–the only thing that PDO does–is highlight those teams who are getting better or worse results than they ought to be given how skilled they are. That’s all it does. That’s all it’s meant to do. It does that very easily and very efficiently.”

You determine PDO by adding a team’s save percentage & their shooting percentage’s together.  Basically what rate of shots on target are saved by the goalie & what percentage of a team’s shots on target beat the opponent’s keeper.  The numbers naturally gravitate towards 1.000.  So far this season, Rangers have a save percentage of 0.765 & a shooting percentage of 0.431 – combine those numbers & you get a PDO of 1.195.  Saturday’s opponent, Falkirk, has a PDO of 1.041 (0.684 SV% + .357 Sh%).

Austin continues – “The greater the distance from 1.000, the more likely there will be a change.  If it’s low, it will go up.  If it’s high, it will go down. This is called regression to the mean, and in the NHL (or any hockey league), the mean is exactly 1.000.  Always.  By definition.”

“Every shot (on target) in the league is either a goal or a save.  There aren’t any shots (on target) in the league that are not goals or saves.  Thus league save percentage plus league shooting percentage is always and invariably 1.000. The average, then, is mathematically defined.”

“The value of PDO lies in its ability to show how far away a particular team’s experience is from the average.”

The current leaders in PDO are Rangers & Raith Rovers, while the teams with the lowest PDO are Alloa Athletic & Livingston.  Now hold that thought as you wonder what that means, because are Rangers really going to regress & are Alloa & Livingston really in any position to improve their play?

Let’s put these results in context by comparing them to another statistic, one that will measure which teams are playing some of the best football…

Lee Wallace & Martyn Waghorn

Expected Goals measures how many goals a team &/or player should be scoring based on the volume & location of shots that they are taking.  This number is explained in more detail here & is a very effective tool to measure which teams are playing positive football that should lead to more goals being scored.  Score more goals, win more games.

Dutch football analyst, 11tegen11has begun bringing the two statistics (PDO & xG) together to measure which teams are succeeding because they’re lucky, while determining which clubs are beating the odds because of skill, not luck.  He explains, “Better teams create more shots than they concede, but they also create better shots than they concede. Part of this is due to quality – creating better shots through better play – but another part of this is due to Game State effects.”

“It’s easier to score then you’re already leading because the defending team needs to take more risks to try and salvage something from the game.”

If you think of Rangers in that context, you can see why their PDO is so high & why it may stay that way.

Let’s look at PDO in correlation to Expected Goals for Scottish Championship teams.

image (1)

In this chart, PDO is going along the vertical axis & total Expected Goals is along the horizontal axis.

Some observations:

  • Rangers dominant play has not only meant that their Expected Goals are far & away the best in the league (more than five goals better then the next best team) but the fact that they have led in each game has led to the creation of far better shooting opportunities then they are conceding.  This would suggest that Rangers will likely not see their PDO regress to the expected mean of 1.000 – they really are that much better then the rest of the field.
  • Hibs & Morton have both played positive football this season & it should continue.  Hibs have struggled to an extent to maximize their results, but in the long run should end up up second to only Rangers in the league table.  Morton are enigmatic, given that their Expected Goals Against is the second worst in the division.  They are creating quality scoring opportunities, but also are giving them up at an alarmingly (yet entertaining rate).
  • St Mirren, Raith Rovers & Falkirk are not playing a style of football that is generating large numbers of quality chances (hence their low xG stats) & quite frankly may be lucky to have earned the results they have so far.  Falkirk & Raith Rovers are currently tied for second in the table, & these numbers suggest that they may want to enjoy their place in the standings while they can.  However, of the two Falkirk may be the one who can sustain given their stout play defensively.  They allow an average of 0.94 xG per match, while Raith allows 1.70 xGA.  St. Mirren fans should just look away…the numbers suggest that they are actually lucky to have six points in eight matches.
  • Queen of the South, Livingston, Dumbarton, & Alloa Athletic have low PDOs & low Expected Goal totals.  Those two numbers combine suggest that, as of now, there is little evidence that they will improve significantly anytime soon.
  • Currently, there are no teams that are what you would term ‘incredibly lucky’, i.e. having a high PDO & a low Expected Goals total.  So the tables should shake out to Hibs rising to the best of the rest & Morton having the potential to compete for a play-off spot if they begin to limit the shots that they are giving up, while still maintaining their style of play.  If Falkirk continues their defensive approach to matches, they will likely also be in the mix for a play-offs.”

You can follow Rangers Report on Twitter @TheGersReport



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