There are a number of different ways to approach this problem, from a mathematical/statistical point of view.
I'll take a simplistic approach, to keep the explanation and maths simple.
Let's say there are 3 easy games, 3 medium games, and 3 hard games each week.
We will allocate a notional points range of 60, 80 and 100 to each of these games.
ie, in an easy game, you might say for example, Fremantle will beat North at Paterson by between 10 and 70. In a tough game, you might say GC could beat Essendon by 30 points, but could lose by 70 points at Metricon. You get the idea. I should point out, by easy game, I am not necessarily talking about it being easy to pick the winner, but easy to narrow the margin down. As an example. You might be looking at Sydney V Geelong at the SCG, and you can be reasonably confident the game will finish between Sydney by 35 and Geelong by 25 points.
Now lets say that most people will pick closer to mean/average of those ranges, when doing there selections, and the result is twice as likely to be within middle part of the points range.
Sounds complicated, but what I am saying is this: Using GC v Ess again.
The results is a 16.66% chance to be GC 6 - 30pts, a 66.66% chance to be GC 1 - 5pts, a draw, and Ess 1 - 45pts, and a 16.66% chance to be Ess 46 - 70pts.
Most people's selections, and most results will occur in that middle 66.66% chance range.
If we take a basic approach, and say each point margin has the same chance of occurring:
then in a tough game the average tipster is a 66.66/50 = 1.333% chance of nailing the margin.
in a medium game the average tipster is a 66.66/40 = 1.667% chance of nailing the margin.
in an easy game the average tipster is a 66.66/30 = 2.222% chance of nailing the margin.
This averages down to the average punter being around a 1.741% chance of picking the winner, and margin, in any randomly selected game.
If you are only selecting margins on a Friday night game, it means you are only trying 23 times/year. The percentages boil down to this:
0/23 margins and winners correct - 66.77% of the seasons you tip in.
1/23 margins and winners correct - 27.21% of the seasons you tip in.
2/23 margins and winners correct - 5.30% of the seasons you tip in.
3/23 margins and winners correct - 0.66% of the seasons you tip in.
So your friend is in the finishing stages of a once in 19 years tipping performance.
Let's put it another way. Ask 20 friends/co-workers how they are going with their margins this year. As we are roughly 70% of the way through the season, you are likely to have 3 or 4 say they nailed one spot on, and your friend with the 2 right. The rest should say they have none.
If you tip a margin in all 198 home and away games for the season, you are likely to pick 3 or 4 margins and winners correctly.
A good way of verifying if my percentages are close to the mark, is to compare them to the odds that are bet on picking margins.
The
TAB bet on 2 point margins. Generally around the "popular" margins the
TAB bet
$26. The
TAB usually set markets on exotic betting on somewhere around 130-135%. $26 represents 3.846% of the betting market. As it is a 2 point bet, we can halve that, so that becomes 1.923%. We now need to divide that by the percentage the market is set to. Let's use the average 132.5, so 1.923/1.325 = 1.451%. So the
TAB would seem to confirm that my "rough" calculations are within the ball park!