At this point of the year it's important that we try and establish what numbers can be believed. In other words: how small is too small for a sample size? Consider, for example, that through six gameweeks last season:
We could go on and on. The point being, six games is not enough to write teams off or buy into short lived success. When then, is long enough? Using data from the past two seasons (ideally we'd use a much larger sample but I don't have it to hand and, honestly, I don't want to to the leg work to get it in the right form), we can plot a teams goals scored/conceded per game on a weekly cumulative basis against their final attacking/defensive record and then try and locate when the GPG rate becomes sufficiently predictive (I'm arbitrarily setting this at 75%). Let's look at the trends:
The first thing to note is that defensive goals per game seem to stabilise at a similar rate whether at home or away, gaining 80% correlation, somewhere around gameweek 12. That said, even at the stage we're currently at this year, it seems we have a decent set of data to predict year end results. That suggests that there is a pretty good chance that the likes of West Brom and West Ham are for real, while formerly great defenses like Liverpool and City might continue to have struggles. Of course, as noted in the introduction, some of these trends will reverse as, if the correlation holds, they're only ~65% predictive, but even so, we're already at a point where one can't just simply say that City are great and West Ham are terrible simply because "that's the way it is".
The second point to note is that for goals scored, the results at home seem to be much more reliable than those on the road. Indeed, we see close to 80% correlation after just seven gameweeks at home while away data is all over the place, only settling down into the second half of the year. Without looking deeper into the numbers I'm not really sure why this is, other than the obvious fact that it's hard to score away from home so your denominator is smaller when calculating GPG. Even one goal swings in that scenario will therefore have a large impact on this analysis. As a side note, we also observe that the r-squared of the home scoring data stabilises at 70% as early as gameweek 10, which suggests that not only is this data predictive of the overall picture, but it also applied to a decent proportion of teams. This suggests that teams like Fulham, Southampton and West Brom could, and possibly should, continue to have success based on their performances to date.
So what does this mean? For the next week or so, not too much. I'm still not ready to give up on City's defense and I'm still not willing to believe that Fulham are an elite attacking side at the Cottage. We should however be looking out for the below (approximate!) dates, at which we can assess where team's stand and, with some certainty, will finish up:
- Aston Villa ranked 10th in goals scored at home and 3rd on the road. They wound up being the 2nd worst attacking side in the league, managing to notch just one goal more than Stoke.
- Tottenham placed 19th in home defense, shipping an average of 2.5 goals per game. They finished the year as one of the league's best defenses, ranking 4th at home and 5th on the road.
- QPR conceded just one goal in their first three away games. They went on to concede 40 more in the next 16, at an alarming rate of 2.5 a game, enough to rank behind everyone outside of lowly Blackburn.
We could go on and on. The point being, six games is not enough to write teams off or buy into short lived success. When then, is long enough? Using data from the past two seasons (ideally we'd use a much larger sample but I don't have it to hand and, honestly, I don't want to to the leg work to get it in the right form), we can plot a teams goals scored/conceded per game on a weekly cumulative basis against their final attacking/defensive record and then try and locate when the GPG rate becomes sufficiently predictive (I'm arbitrarily setting this at 75%). Let's look at the trends:
The first thing to note is that defensive goals per game seem to stabilise at a similar rate whether at home or away, gaining 80% correlation, somewhere around gameweek 12. That said, even at the stage we're currently at this year, it seems we have a decent set of data to predict year end results. That suggests that there is a pretty good chance that the likes of West Brom and West Ham are for real, while formerly great defenses like Liverpool and City might continue to have struggles. Of course, as noted in the introduction, some of these trends will reverse as, if the correlation holds, they're only ~65% predictive, but even so, we're already at a point where one can't just simply say that City are great and West Ham are terrible simply because "that's the way it is".
The second point to note is that for goals scored, the results at home seem to be much more reliable than those on the road. Indeed, we see close to 80% correlation after just seven gameweeks at home while away data is all over the place, only settling down into the second half of the year. Without looking deeper into the numbers I'm not really sure why this is, other than the obvious fact that it's hard to score away from home so your denominator is smaller when calculating GPG. Even one goal swings in that scenario will therefore have a large impact on this analysis. As a side note, we also observe that the r-squared of the home scoring data stabilises at 70% as early as gameweek 10, which suggests that not only is this data predictive of the overall picture, but it also applied to a decent proportion of teams. This suggests that teams like Fulham, Southampton and West Brom could, and possibly should, continue to have success based on their performances to date.
So what does this mean? For the next week or so, not too much. I'm still not ready to give up on City's defense and I'm still not willing to believe that Fulham are an elite attacking side at the Cottage. We should however be looking out for the below (approximate!) dates, at which we can assess where team's stand and, with some certainty, will finish up:
- Home goals scored: Gameweek7
- Away goals scored: Gameweek 21
- Home goals conceded: Gameweek 13
- Away goals conceded: Gameweek 12
Try and keep these dates in mind before you buy you third Everton midfielder/forward or you build your defense around a pair of West Ham defenders.
No comments:
Post a Comment