PDO, regression and Newcastle v Wanderers

Before Newcastle beat the Wanderers 5-0 and render this post entirely useless, here is a quick look at PDO and why I’m keen on referencing it. It is by no means a perfect metric that can explain everything. But it does have its uses.

PDO is the sum of a side’s Score% and Save%. Each calculated as:

  • Score%: Goals For / Shots on Target For
  • Save%: 1-(Goals Against / Shots on Target Against)

Over the course of a season, most side’s convert their accurate shots into goals at a similar rate. And likewise, they save them at a reasonably similar rate at the other end. There are outliers of course – Melbourne Victory had a Score% of over 44% last season – more than 6% better than the next best effort over the last three years. The majority of sides will have a Score% between 25% and 35%.

From a  defensive perspective, only 2 out of 30 sides in the last three years hasn’t had a save% between 63% and 75%.

So why does this matter? There is a luck in football. Because games are decided on such small margins, it is possible for a team to perform in the short-term, and win games, at a level that is not sustainable over the rest of the season. At the start of the season, a side may convert a high percentage of their chances in front of goal, but over the course of the season their conversions will regress to the mean. Converting 50% of your chances for the first month, does not mean you will for next 23 rounds.

This weekend’s A-League clash is a perfect example of team’s with contrasting fortunes so far. We are four weeks in to the season and Newcastle find themselves in equal second position on the table. Scott Miller deserves credit, he has taken a team that won only three matches last season to as many victories in four weeks this time around. They look more defensively secure and the players are on the same page as each other.

But. And it is a big but. They’ve saved 83.33% of the shots on target they’ve faced. Only the Wanderers’ remarkable debut year comes close (Save% of 80.5%). It’s unlikely they can sustain this over the season. They could turn out to be one of the most defensively sound teams we’ve seen and they certainly restrict side’s from taking good chances. But to keep saving at 83%? I’m not convinced.

Not only have the Jets rode their luck in a defensive sense, they have also managed to score from 60% of their shots on target.

You can point to the resolute, compact defence as being hard to break down, or the fact the chances they will create are on the counter and often at an exposed opposition keeper. But to put it in perspective.. The last three year’s in the A-League look like this:

Screen Shot 2015-11-07 at 12.46.13 AM

At the other end of the spectrum though is Western Sydney. They so far have a score% of just 16.67% and a Save% of 28.57%. Things can only get better from here. They are ranked #1 in the competition for Total Shot Ratio – taking 68.54% of the shots in their matches and #2 for Shots on Target Ratio and Shots Inside the Box Ratio. They are dominating shots, but with little luck in either box.

Newcastle are the 2nd worst side in the league in all three of those metrics.

Score% and Save% takes a few weeks to settle and regress and this Saturday’s clash could be a starting point. See this graph here (press play bottom left corner) to see how each team’s PDO has evolved throughout the season over the last three years.

To see an interactive version of the above image click here.

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