All these H2H data are very interesting, and nicely put up by AlanY.
BUT, are they relevant at all?
I mean, the recent H2H's are obviously relevant, but the older ones? How much weight would you put on a match played say 5 years ago?
In particular I think they are not much worth, if you compare players of different ages, because you would then include matches where one player is in his prime, while the other is still very young (Players like Axelsen and CTC), OR the opposite, a player that was in his prime while the H2H stats was compiled, but now is getting old (say Tien Minh Nguyen and Boonsak Pontana).
Look at the upcoming semi-final between Chen Long and Axelsen.
CL is 5 years older than VA. Today, at ages 22 and 27 that difference might not be so huge. But their H2H record is compiled while VA was 18-21 years old, and CL was 23-26. The younger you are, the more a 5 year difference will affect the outcome.
CL is 7-1 over VA in their H2H, but VA won the latest match. The latest match should obviously carry most weight. And another of the latest matches (their WC clash last year, had CL winning by 30-29!!!).
And the matches they played back when VA was merely a junior, it's no surprise that he lost them to his 5 year older opponent.
So 7-1 probably isn't 7-1 when you look careful at the numbers. H2H data would suggest to me, that CL is more like a 2:1 favorite (3:1 in matches last 18 months, with VA winning the latest which I then count as 1.5).
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With the players that have gone old and leave behind a H2H track record they surely cannot match anymore, the data is like this:
Chen Long has lost two matches to Boonsak, and one match to Nguyen, so they are capable of beating him, you could think.
No they are not. Boonsak won the two first matches he played vs. CL, back in 2010, where CL was only 19 years old, and Boonsak was in his prime. Since then, CL has won all matches, and Boonsak is now no longer the player he used to be. There's no chance in hell he will ever win a match against CL again, unless CL has to forfeit.
For Nguyen its the same picture. His sole win over CL was 4½ years ago. Back when CL wasn't fully matured, and Nguyen could still move around the court at high speed. Nguyen is a player relying heavily on his physique and stamina to win matches, so he is really feeling an impact as a result of his age, compared to other players. Again, there's no way CL could ever loose one more match to Nguyen, even though the H2H stat say it's possible.
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Therefore, when doing H2H comparisons, I suggest you:
1) Limit the scope to a few years, especially if there's a larger age difference between the players, and in particular if one of the players is a young player.
2) Put more weight on the latest matches, especially the last match.
3) Put more weight on matches played in big tournaments, where we know that both players are trying to peak! (or remove the "practice matches" from the statistic).