Cork City: Resilient and Clinically Efficient or Lucky.

PDO was a concept developed in Hockey and recently brought over to soccer analysis.  Simply it is a teams shots scored percentage plus their shots saved percentage.  This figure is usually multiplied by 1000 just for presentation, to remove decimal places. 

(goals scored/shots on target + (1 -  goals conceded /shots on target faced)) * 1000

A high PDO means that you are either scoring a high proportion of your chances or stopping a high proportion of your opponents chances or both.  Either way a high score indicates you are performing well. 

What the analytics people find interestesting about the PDO is that even the best teams cannot sustain a high PDO.  Studies show that short term peaks in PDO level out over a season.  "Normal" values of PDO range from about 980 to 1020 according to the research.  Values outside this range usually indicate that a team is over performing (or lucky) and their PDO is fully expected to regress towards the mean in the long term.  As such, some analysts study the PDO to understand whether a team has been "lucky".

The Pythagorean expectation is a concept borrowed from Baseball.  The formula is:

win per cent = (runs scored ^ 2) / (runs scored ^2 + runs allowed ^ 2)

It gives the number of wins expected based on a teams home runs scored and home runs allowed.  Obviously the more home runs you score, the more games you expect to win.  In soccer a number of equivalent concepts exist.  The difference between the Pythagorean win expectation and the actual win count is often put down to good/bad luck (some days when you need them the goals just don’t go in, other days when maybe you don't need them as much you have a goal bonanza).   Hamilton developed this formula (which accounts for draws, something not heard of in Baseball) but a simpler version that we will use is documented in this blog article.   

The below table shows the League of Ireland as it stands, with both PDO and Pythagorean expectation calculated.  Cork City score very high on PDO which the analysts would have us believe that they are performing above their normal standard or any sustainable standard (maybe due to luck, or something else?).  In a single game against Athlone their PDO was the maximum possible at 2000 (they scored every shot on target they had at the same time as keeping a cleansheet).  The match report is here.  In another game Cork won by a single goal against Derry but were on the defensive for most of the game.  The report is here.

 

Cork have also (luckily?) accrued many more points than the Pythagorean expectation would have us believe that they deserve based on their goals scored.  If you look back on their results, you notice a high proportion of their wins (13) were by a single goal (and they were outshot in 5 of those games).  Compared to 8 single goal victories for Dundalk.  This is leaving things a little too tight for comfort - but they got away with it.  Dundalk were often much more comfortable in victory winning by 7 goals, 5 goals and on two occasions by 4.

Dundalk also have a fairly high PDO score (at least outside what I am supposed to believe is the normal range).  Are we to believe then that there is a very high proportion of luck involved in the League of Ireland.  Or do different rules apply - maybe a higher level of PDO is sustainable in Ireland because of the mixture of teams in the league.  There is a wide spectrum from the top teams who compete in European competition to the lower table teams who are really just a small step up from the semi pro/amateur level of the First Division.

The Pythagorean expectation is within a point or two for most of the teams including Dundalk - there are some big differences though in the case of Athlone, Derry and Cork.  Take Athlone for example, they out scored UCD and Bray and also conceded less than both, but unluckily find themselves rooted at the bottom after drawing 9 and losing 9 more by a single goal.  Unfortunately for them their goals and cleansheets came when they needed them least.

But lets focus back at the top, and in particular the top of the table decider where Dundalk host Cort City on Friday.  Will Cork's luck finally run out on the last day of the season.  Or can Cork avoid defeat to prove that there was no luck involved - but rather resilience, clinical efficiency and an impenetrable last line of defense.