Just a short question (I admit I am too lazy to look it up in the ranking system description):

Does it make a difference if I win (or loose) against a strong or a weak opponent of a certain rank?

That would mean that a win against of a strong 5 kyu (almost 4 kyu) counts more than a win against a weak 5 kyu (who just made it across the barrier from 6 kyu).

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Stay out of my territory! (W. White, aka Heisenberg)

I think so. As I understand it the system works on the internal non integral rank.

It also matters if your opponent's rank changes after you play him.

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Sanity is for the weak.

Li Kao has it right. The system internally uses a floating point rating that you can see in the rank charts for users. Then this floating point rating is converted to a rank like "1d" that is shown to you. But internally, it stays floating point, so a strong 1d is counted as stronger than a weak 1d.