Thursday, May 24, 2007

Goal difference vs Head-to-Head - who benefits

This is a question that has bothered me on and off for a while now. Personally, I prefer the goal difference comparison - if only because you can get all the information you need from a simple table of standings (and don't have to go to a list of all the results to see who won).

Still, that's not a really good reason for preferring one method of 'ranking' over another. So, time for some meaningless numbers.

I've got this spreadsheet thing that can randomly generate a set of results for international football matches - with the likelihood of a result based on FIFA ranks and the typical relationship between past international results and FIFA ranks (now, there's a long story). If you run a lot of random trials (say, 50,000) for a group, and compare who would finish where based on GD and HTH rankings, you can get an idea of what the effects are.

So, I've done a number of different scenarios, based on a 4-team group playing a full 6 match schedule. To make it look vaguely realistic, the groups are of 4 African sides, using May 2007 FIFA ranks.

In each case, I've put in a table showing:
  • Team
  • FIFA rank
  • % times finishing 1st under each method
  • % times finishing 1st or 2nd under each method


Which gives a handle on what we might actually care about as a result of the group.

TRIAL ONE - A mixed group - no hopeless teams, but a range of abilities.






TeamRankAve PtsAve Pos GDAve Pos HTH%1st GD%1st HTH%1-2 GD%1-2 HTH
Cameroon96813.11.341.3673.7%72.6%93.1%92.7%
Guinea5939.32.282.2917.2%17.8%62.8%62.4%
Togo4667.62.722.728.2%8.6%36.9%37.2%
Liberia2244.03.653.630.9%1.0%7.2%7.7%


TRIAL TWO - One good team - three no hopers.






TeamRankAve PtsAve Pos GDAve Pos HTH%1st GD%1st HTH%1-2 GD%1-2 HTH
Cameroon96816.81.011.0198.9%98.7%99.9%99.9%
Guinea-Bissau187.52.582.590.8%0.9%56.6%56.0%
São Tomé e Príncipe05.13.213.200.1%0.2%21.8%22.2%
Djibouti05.13.203.200.2%0.2%21.6%21.9%


TRIAL THREE - Three evenly ranked sides - one no-hoper






TeamRankAve PtsAve Pos GDAve Pos HTH%1st GD%1st HTH%1-2 GD%1-2 HTH
Egypt69411.02.002.0133.8%33.8%66.7%66.8%
Senegal69211.02.002.0033.7%33.7%67.0%66.8%
Mali68510.92.032.0332.4%32.5%66.0%66.0%
Somalia131.53.973.960.0%0.0%0.3%0.4%


TRIAL FOUR - One top side - three others around the same






TeamRankAve PtsAve Pos GDAve Pos HTH%1st GD%1st HTH%1-2 GD%1-2 HTH
Cameroon96810.81.761.7753.1%52.3%78.5%77.9%
Egypt6947.72.722.7216.2%16.5%41.3%41.5%
Senegal6927.62.752.7415.6%15.9%40.6%40.9%
Mali6857.52.772.7615.1%15.3%39.7%39.8%


TRIAL FIVE - Two good - two bad






TeamRankAve PtsAve Pos GDAve Pos HTH%1st GD%1st HTH%1-2 GD%1-2 HTH
Cameroon96814.21.421.4259.2%59.0%99.3%99.1%
Côte d'Ivoire85013.41.611.6140.7%40.9%98.8%98.5%
Guinea-Bissau183.73.463.460.1%0.1%1.1%1.3%
Somalia133.43.523.520.0%0.1%0.8%1.0%


TRIAL SIX - One good - two middling - one poor






TeamRankAve PtsAve Pos GDAve Pos HTH%1st GD%1st HTH%1-2 GD%1-2 HTH
Cameroon96814.11.261.2779.0%77.9%95.2%94.9%
Togo4669.32.402.3910.5%11.1%52.4%52.5%
Zambia4629.32.412.4010.5%11.0%51.8%51.9%
Somalia131.83.943.930.1%0.1%0.7%0.8%


Pretty much overall, GD gives an advantage to higher ranked sides. Except where ranks are very close (so randomness comes into play), the GD ranking is more likely to see the best ranked side finish higher - or the best two sides finish 1-2.

Of course, there's millions of things that make this unrealistic, but the hope is (I suppose) that by looking at an average of 50,000 trials these are sort of smoothed away. However, I think the results are clear, and match what I might have expected, and match what happens in reality - namely that lower ranked teams tend to get an advantage from using HTH as the tie-breaker.

So, if you prefer the 'best' teams to make the finals, you're probably going to want GD used to rank, if you prefer 'surprises' (Angola ahead of Nigeria for example) then HTH is more your thing.

Saturday, May 19, 2007

In the beginning...

In the beginning there were deadlines. And the deadlines slowly approached. And the deadlines even more slowly focussed the mind...

Sadly, the mind became focussed on things that - to others (let us call them, for want a better title, "employers") - were of less than earth shattering import.

Yet focussed the mind became.

Questions emerged. Such as "is there a solid relationship between FIFA world ranks and international match results", "is head-to-head really as bad a method of tie-breaking as I think it is", "if you went back to the first FA Cup match and tracked the winners through the thousands of games since - who would you be following now" and "how much more likely is Australia to qualify for the world cup now they qualify out of Asia rather than against the last number Sepp Blatter thought of".

Some of these questions involved numbers.

Some of them involved vast arrays of data and spreadsheets and stuff like that.

Some may even involve answers...