In our Halfway Point series of posts we have already looked at both scoring % and save % and now we move on to what is a combination of the two: PDO. Its premise is thus: PDO = scoring % + save %. PDO is the combined total of a teams ability to keep the ball out of their own net and the ability to put the ball in the opponents net.
We know that PDO is scoring% + save%, but what does it tell us about a team if the number is high or low?
A high number indicates excellency in keeping the ball out of the net (save%) and/or putting the ball in the opponents net (scoring%). A low number can indicate a poor ability to score or to save. Some teams have a high PDO on the back of an excellent scoring% (Stoke) and an average save%. PDO, in essence, is a marriage of these two abilities to give us a wider view of a specific team in terms of looking at defensive and offensive ability.
Is PDO a good measure of a good team though?
The best teams in terms of league position didn't necessarily have the highest PDO number. However, there are other factors to league position than just scoring % and save % (thus PDO). A high volume of shots on target for, for example, can mean a lower PDO but indicate a good team who scored lots and who finished high in the standings. Conversely, a team which conceded few shots on target against and conceded few goals against also can have a lower PDO but still excel in terms of points.
Overall, good teams will score at a higher rate and make saves at a higher rate but this isn't a revelation. There is an element of luck - due to the data sample size - when using PDO for a one off game, but when looking at a bigger body of evidence - even a half season (although still a smaller sample size than is ideal) - that luck should even itself out. A team with a high PDO doesn't make it a great team, it just shows us who was efficient in converting the number of shots on target for into goals, and who was efficient in keeping out the shots on target against.
We also have to understand that not all teams have the same quality of players at their respective clubs. United will always likely have a PDO above 100 and a relegated side will always be in the 90's. No relegated side would have posted a very high PDO number as if that team had posted a high PDO number over the course of a season they would almost certainly have not have been relegated.
PDO through 19 games
Taking the bottom end of the table first, we can see that it is no surprise that both QPR, Aston Villa, Wigan, Southampton and even Newcastle have struggled over the first half of the season and face a very real battle to avoid relegation.
At the top it is no surprise to see United riding high but Chelsea are an interesting case and lead the way after 19 games. They have been consistent both in terms of converting (second only to United) and third in terms of save %, a performance in both areas that no other side has managed. However, rather than having an excellent percentage in one or the other category they have been very good in both, which goes some way to explaining them lagging behind the Manchester clubs.
And we can also compare the halfway point of the season with the final PDO table from 2011/12 to see come continuing trends: firstly with Swansea and in particular, Stoke, all above the 100 mark whilst West Brom and West Ham have high figures to sustain top half placings at half way.
At the wrong end once again are Liverpool, whose scoring % sees them ranked 13th but whose save % of 61.76% means they are rock bottom in this category. What they have done this season is improve their scoring % but their progress is still being hampered by their inability to keep the ball out of the net in relative terms to the amount of shots on target faced.
Teams of interest
Here on Bitter and Blue we have long held Stoke up as a club who provide a huge amount of interest in terms of the numbers. We can see from their 19 game moving average they have been on an upward trend from week 10 onwards. Their season has been based upon an excellent defensive record, restricting opponents from shooting and a clinical nature in terms of converting the relatively low number of shots on goal. If they continue this trend over the second half of the season it should see them once again challenging for the Europa League positions.
And on to the Manchester clubs. We know that United held a seven point lead at halfway and has been above 100 since week 2, even more impressively has been above 110 in 12 of the 19 weeks; little wonder they have built the lead they have. City have struggled for the most part, in comparison to United's totals they have been above 100 for just one week of the season.
For the title race to be a close one as we head to the final weeks of the season, the next dozen or so games will need to see a reversal of both City and United's totals. Will this happen? United should regress towards 100 but will it be severe enough for City to then improve and take advantage? Unlikely, but something to watch as the season progresses.
We touched on some of those teams at the bottom end (particularly Liverpool) and here we can see the bottom three over the first half of 2012/13 on a moving average basis. Again, five or six games in provides a consistent starting point basis and from there we have seen a slight improvement, but worrying for QPR and Villa in particular in terms of the spectre of relegation:
For Villa in particular, their low PDO is another sign of the problems they have faced over the first half of the season given that we have touched already on their strength of schedule and injury problems. Many may feel that they could be a classic 'too big to go down' club but on the evidence seen so far, they are in big trouble.