The relationship between points and goal difference

In a previous post I put

A team that has a goal difference of 0 will pick up an average of 24-25 points over a period of 18 games and this is increased/reduced by ~0.6 points for every goal above/below 0 that a team is. So if Lincoln perform as expected then they should pick up about 21 points
The surprising form of Lincoln City – part 4

Below is a plot of points vs goal difference for the last ten years in the premiership. (I’ll later put up a similar plot for the last ten years in the football league but I’m willing it to bet it looks remarkably similar)

Photobucket

The correlation is very strong (not exactly a ground breaking finding – whether or not you earn points depends on your goal difference in a game).

So quickly for the example of a team having a goal difference of -6 over 18 games:

Exp points = (18 X (52.06/38)) + (-6 X 0.65) = 24.67 – 3.91 ~ 21

To put this into some context over a full season the table below shows the expected points for a team that finishes a season with a particular goal difference. I’ve split it into 38 and 46 game seasons to represent the premiership and lower leagues.

Goal difference Exp points (38 games) Exp points (46 games)
60 91 102
50 85 96
40 78 89
30 72 83
20 65 76
10 59 70
0 52 63
-10 46 57
-20 39 50
-30 33 44
-40 26 37
-50 20 31
-60 13 24
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