Relative Shot Rates and PDO in the English Premier League

logo eplWith the Dutch Eredivisie taking its usual winter break, the opportunity arises to apply some of the most promising metrics in football analysis to other leagues around Europe. The driving force behind this initiative, as is true for most of the work on this site, is curiosity. In this case, it is curiosity to compare the Eredivisie to other leagues in terms of Relative Shot Rates (RSR) and PDO. But before we come to such comparisons, let’s study the findings in other leagues, starting with the most prominent league in the world, the English Premier League.

For those unaware of the terms RSR and PDO, let’s start with the latter for a short summary. A more extensive description can be found in the post on separating luck and skill, which introduced the concept of PDO on this site. The term PDO is adapted from ice hockey analysis, where it was introduced by Brian King (whose internet alias happened to be ‘PDO’) and picked up by James Grayson, who first applied it to football and has taken it further from there.

PDO is the sum of a team’s saves percentage and shot percentage, where saves percentage is the fraction of conceded shots that don’t result in a goal, and shot percentage is the fraction of shots created that results in a goal scored. For convenience, PDO is multiplied by 1000 to get rid of the decimals.

The RSR is an extension to the TSR, which stands for Total Shot Rate. A team’s TSR is computed as the fraction of shots created from the total number of shots in all matches played by the team. The RSR is a slight adaptation, which compares a team’s number of shots created and conceded with the league average against the same opposition. More details on the method behind TSR and RSR are found here.

Without further ado, here’s the EPL league table, updated with Match Day 22 results, including RSR and PDO. Remember, a high RSR signifies a relatively high ratio of shots created, and is a strong characteristic of sustainable good performance. A high PDO signifies a high ratio of shots converted and/or saved, which has proven to be a lot less sustainable over the longer term.

In general, high PDO teams are found in the top half of the table, and low PDO teams in the bottom half. The team with the highest PDO (1062) is Manchester United, mostly due to their immense conversion rate of 17.4%, which is over 50% better than the league average of 11.1%. The other exceptionally high PDO is Chelsea, but also Stoke, West Ham and Swansea punch above their weight, with PDO’s at a level that could only be sustainable by top teams. From a recent long term PDO analysis, we’ve learned that PDO’s outside of the 980-1020 zone seem unsustainable beyond the scope of a single season, while inherent differences in team quality may account for variations within this zone. So, we may expect Manchester United, Chelsea and to a lesser extent Stoke, West Ham and Swansea to drop off a bit in the remaining part of the season.

Remarkably low PDO teams are clustered near the bottom, where all of Newcastle, Aston Villa, Southampton, Wigan and QPR look set for an improvement on their points-per-game haul so far. Also, Liverpool and Tottenham rank low in terms of PDO, which means an improvement in terms of points-per-game is just around the corner.

 

RSR

In terms of RSR, there is quite a clear top-3, with Manchester City, Liverpool and Tottenham the only teams above the 0.600 mark. This means that, with hypothetical equal conversion rates, these three teams would be in a close fight for the title. And, since shot rates are a lot more sustainable on the long term than conversion rates and saves rates, these three teams reflect the best underlying performance level. Behind them, Everton and Arsenal are a close 4th and 5th, with Chelsea at 6th place. Perhaps remarkably, league leaders Manchester United come in just 7th in terms of RSR. This indicates that Sir Alex Ferguson’s team is highly reliant on substantially higher conversion and/or saves rates, which seems a precarious base for future success. However, so far, their exceptional PDO has earned them a gracious seven point lead over rivals Manchester City, which may well be enough to win the league.

Based on these parameters, the top-3 will most likely be United, City and Tottenham, with a close battle for fourth between Chelsea, Everton, Arsenal and Liverpool.

A the bottom of the table, both RSR and PDO spell doom for recently promoted Reading. They are the bottom team in terms of RSR, and by a distance, but their PDO of 1017 indicates that their shots and/or saves percentage has been above the average EPL level, which is more than can realistically be expected of this side. A PDO at the low side of the 980-1020 zone seems more realistic and a disconnection with the pack battling for survival seems imminent for Reading.

6 thoughts on “Relative Shot Rates and PDO in the English Premier League

  1. bart

    Your conclusions about PDO are only half correct … here’s why …

    If we take your last post on how PDO moves over time/games we have to compare the teams’ PDO with those in the graph after 21/22 games … yes after 38 they will eventually reach around the 980-1020 mark, but after 21/22 games the teams should be within 955-1040 range … your graph shows that a team with 1020 PDO after 38 will be around 1040 after 21 games … so you’ve basically got your conclusions correct, but your argument of why is incorrect …

    e.g 1: Chelsea’s PDO of 1058 should actually be around 1040 (as they are in Q1 of the league table) still too high, but not as bad as comparing 1058 to 1020.
    e.g. 2: Fulham’s PDO is 1007, which for a Q3 team is actually too high, even though it looks smack on target … after 21 games Q3 teams have a 985 PDO on average which will be at 1000 after 38 games … therefore one can expect Fulham’s “luck” to drop more drastically than you indicate, they are actually riding their luck a bit (on top of that their RSR is quite low in the league rank)
    e.g. 3: Whereas the team just below them, Newcastle, has a lower PDO (956) than expected after 21 games (980) AND their RSR is quite high …

    One might thus expect Newcastle to finish higher than Fulham in the final tables if we take these numbers into account.

    So … you’ve got two posts that are in principle good but you miss out on highlighting some interesting teams like Fulham … they look good when you just look at the individual numbers … almost 1000 PDO, near 0.500 RSR, joe average … but when you take your previous post which shows how PDO develops over a 38 game season they kinda look in trouble, or they should be looking down the table and not up (as their points may indicate).
    You’ve done a lot of hard work, you have to flesh the most out of it and not ramble out the standard story each time.

    Cheers,
    b.

    Reply
    1. 11tegen11 Post author

      Thanks for your extensive reply, Bart.

      The point I was trying to make with regard to PDO’s outside the 980-1020 zone is that over time they will move to values within that zone. No more, no less.
      Yes, we’re only just over half way into the season, so one may expect several teams to still be outside the zone just because of random effect, i.e. bad or good luck. But over time, these teams will move back into the zone, probably just within the 38 games that an EPL season has.

      The previous PDO post excluded the highest and lowest values, and from face value, in this EPL sample a range of 960 to 1040 is roughly what you’ll find for this league too. This does not imply that Chelsea’s 1058 should be 1040, but that Chelsea’s PDO is unsustainable over the long term, just as any PDO over 1020 is.

      The Fulham case is definitely interesting. Their 1007 PDO is definitely of a higher conversion and saves rate than you’d expect from a low to mid table team, so values of 990-995 sound more realistic here (i.e. the low part of the 980-1020 zone). Indeed, they should be looking down, rather than up at the moment. As should Sunderland by the way, who have an even lower RSR than Fulham…

      Obiously, there’s a lot of stories to tell from this info and the text of this post does not even begin to capture them all. It’s just more interesting to look at individual teams once I’ve got round to posting home / away RSR’s and offensive / defensive RSR’s…

      The EPL stuff has only just started here, just stay tuned!

      Reply
  2. bart

    Just read your piece in the Volkskrant … ‘PSV benut zijn kansen het beste – toeval of kwaliteit?’ … can’t reply there cuz I don’t feel like making an account for myself … anyway … what I can’t wait for is the exact same graph in week 34 with a “before” and “after” state when you do a retrospective post … i.e. PSV position after 20 games and the with an arrow and dot the the position after 34 games … that might convince some of your readers. 🙂

    cheers for the reply to my earlier comment too.

    Reply
  3. Paul

    I’ve a question regarding the calculation of the RSR. When I understand it clear, the RSR compares the shots a team takes and concede with the average of the shots other teams took (conceded) against a particular opponent (and do this for all the matches played).

    Is it true that this average is also highly dependent of the quality of the teams where the opponent played against (particularly in the begin of the season and after the winterbreak? For example when Zwolle played their first 3 matches against Ajax, Feyenoord and PSV they have a much higher average of shots conceded. When after this matches FC Utrecht plays against Zwolle the average where FC Utrecht’s shots are compared with is “biased” because Zwolle only played against top teams. Is this not a problem for the reliability of the RSR? Hopefully you understand the point I try to make..

    Reply
    1. 11tegen11 Post author

      Thanks Paul, I think I understand your question well.

      The average that teams are compared against changes with every game played.
      For instance, if only one match were to be played, this match would influence the average number of shots for two teams. However, the RSR of all teams in the league changes, because the average of two teams changes.

      So when Zwolle faces Utrecht after a tough season start, initially Utrecht’s RSR will be biased, but as the season progresses, we learn more about the true capacities of Zwolle and this bias disappears.

      In other words, the average that serves as a benchmark is updated with every match played. Not just at the times of playing the match…

      Hope this clears it up for you, if not, don’t hesitate to ask some more explanation or examples…

      Reply

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