@retroshaft: Generally, closed kan's are fine when you are 1-away from tenpai or better, ideally with a good wait. In most cases, an open kan will lower your hand value and not worth it. It's also risky to call kan against someone who has riichi'd. Not only would they get a chance for a second dora, but also for an additional ura dora!
Before calling a closed kan, make sure if that fourth tile can be used in a new meld. For example, if you have 3333 in your hand, you can treat a 3 as a possible start of a new sequence (123, 234, or 345) + a triplet. If you have 4 of any honor tile and you need to fold, it'll buy you 4 turns of safe discards.
Kan's are also good when you are badly in last place. You've got nothing to lose, so might as well take the gamble. Sometimes, the extra dora/ura dora and extra fu can push you into third place.
Also, don't do this, but occasionally, if I'm in the lead, I'll kan if I have if I have 4 dora tiles even if my hand is completely terrible. It's a real power move, and in higher levels, sometimes people will immediately fold.
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