I mean a large number of matches played between the same two teams.

You can compare it to how it is done in medicine. Imagine a match between a drug and migraine. Would we only do a single test to determine if the drug "wins" against migraine? Of course not. We do many tests and determine a p-value. We can do the same thing in soccer.

Now, of course we cannot do this in a real tournament (it would take too long), but we can draw conclusions from such a test, or several such tests.

The problem there is that there is often far too much variation in the teams year on year - squads change as players get bought, sold or retire, managers come and go. And when you do find a pattern I can virtually guarantee that it won't be due to some novel gamesmanship, but rather finance. It is no secret that the most successful teams are those who are wealthy enough to able to buy (and pay) the most sought-after players and coaching staff, and build and maintain the most sophisticated training facilities (and a whole host of other smaller things that cost money). If we take a look at the finalists of the Europa League this year you can see that the money they both spent on their squad dwarfs all of their previous opponents in the competition combined: https://www.reddit.com/r/soccer/comments/1krtgtt/combined_pu... (ideally you'd factor in wages too but that's harder data to get hold of, but it'll paint a similar picture of not a more polarised one).

What is really interesting and far more worth studying is that there are some very fun outliers - clubs like Bodø/Glimt who have a miniscule annual budget but have overperformed in European competition in recent years. They reached the semi-finals of the Europa League - beating Lazio, Olympiacos, Porto along the way - and in previous years have had similarly deep runs where they battered Roma 6-1 and beat Besiktas and Celtic both home and away. All of the teams I mentioned Bodo/Glimr defeating will have an annual budget that is 10 time theirs (or more) and will frequently make high profile international signings, while Bodo spend frugally and have a predominantly (if not all) domestic squad.

I mentioned Manchester United and Tottenham - they're also outliers worth studying, but in another way. They were both utterly woeful in the league last season despite having astronomical budgets.

If you can crack what causes a Bodø/Glimt, a Manchester United or a Tottenham then you'd be a very valuable addition to the backroom staff of any football club with a desire to punch above their weight...