Using Past Performance Data 2
In my ratings for club football I include the last 32 performances. I have found this to be the optimum number, having tested lots of others. Thirty-two games is almost a whole season in the Premiership and Scottish Premier League, and more than two-thirds of a season in the Football League.
The last performance, as I mentioned, is more significant than the one before, but it is not that much more significant. In my ratings, the most recent performance is given only three times as much weight as the one that occurred 32 games ago. In general, large samples of results are more revealing than small ones. I promised earlier that I would describe a couple of ways in which you can assess teams intuitively.
The first involves trying to relate teams to others in their division. If they played for a whole season how you think they are playing now, where do you think they would finish? Would it be in the middle? Higher or lower? How much higher or lower? At the top? At the bottom?
In both the Premiership and the Football League, teams who finish a quarter of the way down a table - fifth in the Premiership, eighth in any division of the Football League - usually have an average goal difference per game of +0.3. That is to say, if you subtract the number of goals they concede from the number of goals they score, and divide the result by the number of games they play, you will usually end up with a figure of around +0.3.
Teams who finish three-quarters of the way down a table - 15th in the Premiership, 18th in the Football League - usually have an average goal difference per game of -0.3. Teams who finish at the top normally have an average goal difference per game of +0.8, those at the bottom -0.8 . In the Premiership, the extremes are even further from the centre - teams who win the Premiership usually have an average goal difference per game of around + 1.1, those who finish last around -1.1.
In theory, a team with an average goal difference per game of +1.1 will win 62 per cent of their games, draw 23 per cent and lose 15 per cent, gaining an average of 2.1 points per game (80 over a 38-game Premiership season). The total number of goals in their games will average 2.8. In practice, these figures will often vary a bit.
You will have noticed that so far I have discussed only games between teams from the same division. What, you might ask, about the FA Cup or the League Cup?
I say: assess a team first within the context of their own division, then allow for the differences between divisions. A Premiership team would normally have an average goal difference per game 1.0 higher if they played in the Championship, 1.7 higher if they played in League One and 2.4 higher if they played in League Two.