I'm currently researching how KGS ranks changed over time. As preliminary analysis of data contradicts my expectations, I would like to ask you for your experiences.

I remember there seemed to be a large event in 2006 or maybe a bit earlier, which changed lower and higher ranks in different directions by a large margin. This was reflected in my data, as dan ranks decreased pretty significantly. However, kyu ranks decreased instead of improved, too -- I expected the opposite.

There was another visible change in 2008, during which all ranks improved significantly, but not permanently. I do not recall this to be the case in reality, though I may simply have forgotten about it.

Do you remember how ranks changed over time? I'd be glad to hear your anecdotes, or to be pointed at documents about this topic.

I jumped from 7k to 4k overnight in the Great Rank Shift of late September 2006: http://www.gokgs.com/gameArchives.jsp?u ... 06&month=9 (I have a saved rank graph somewhere). I do recall another smaller adjustment a few years later. These days I think KGS low dan ranks are a little stronger than they were 5 or so years ago. Also high dan ranks have got tougher: shiryuu was 8d when he played Chen Yaoye back in 2006 and ended up as 6d (though maybe he got weaker), and players like ZChen used to make 9d accounts easily but now he's only 8d (though maybe out of practice?).

Going back even further, I was the first ever 7d on KGS, when it was still young and not heavily used. In the mid-2000s I could occasionally get to 5d. Today I can just about maintain 3d with my infrequently used playing account.

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Still officially AGA 5d but I play so irregularly these days that I am probably only 3d or 4d over the board (but hopefully still 5d in terms of knowledge, theory and the ability to contribute).

I have heard people speak of 'rating drift'.
I understand it is the reason why ratings are more or less regularly adjusted.
You may want to search for rating drift some more.

On a related note: lately, I played several Japanese persons, using KGS automatch. I sometimes had the impression that (some) Japanese test KGS ratings, possibly to compare it to Japanese ratings?

I remember being ~15 kyu then taking a break, I came back after the rank shift in 2006 and I found I was 7k now, I also remember being 30k for quite a while and just looking around after the rank shift beginners don't seem to stay at that rank, although maybe that's just experiential, liberty bot was 23k back then and I remember being proud when I could finally beat it, so that may give some reference for the 20k ranks being squished into 20-15k now