Mahasattva Someone at my local Go club was claiming that KGS ratings are about two stones higher than in reality. I think what he meant was that KGS playing is somehow of a lower standard than in ordinary "over the board" play, and that a high rating is easier to obtain on the KGS than elsewhere.
I am not sure I can agree with this, as it seems to me there are some strong players on the KGS who fully deserve their ranks.
Has anyone got an opinion on this?
It varies from person to person but, overall, I think that KGS ratings are around two ranks stronger than AGA ratings or around the same as EGF rankings. For example someone whose AGA rating is -2.5 (2 kyu) would probably be around 4k on KGS. However, there are people whose Kgs rank is higher than their real world rank. This can happen because they are good fast players since most KGS games are fast games.
Also, some people game the system by various means, such as playing one of their accounts against another, to artificially make their ranking higher. So you can't really make a general translation between KGS ranks and over-the-board ranks.
Rating systems have a tendency to drift, and may drift in different directions at different speeds in relation to each other. Although the makers and maintainers of rating systems try to avoid drift as much as possible, you will find discrepancies between rating systems. That does not mean, of course, that the players in question are actually stronger (or weaker) in another system, you could compare it to measuring temperature in degrees Celcius, Fahrenheit or Kelvin. 0 °C is not colder than 32 °F, even though it is a lower number.
At Sensei's Library, we try to keep an up to date rating system comparison list, which you can find at: Rank - worldwide comparison. It is a reasonable estimate, but it may not always be up to date (eg: sometimes go servers will make rating algorithm adjustments, or do resets, and then the information on that page will get outdated).
The example with measuring temperature seems to fit quite well: it is just different systems. For example in Europe ranks go up to 7d, and on KGS the highest rank is 9d. So if the same bunch of players would play in Europe and on KGS, their KGS ranks should just be too ranks higher - since the system goes two ranks higher!