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East 1, 7th discard, dora is , this is your hand:



would you wait hoping to improve your wait, or declare riichi?

If you draw you gain pinfu and a shot at sanshoku, but if you draw you get only pinfu with 6 waits instead of 4, and there are just 8 tiles that can improve your wait, and just as many that doesn't improve it (2-dot and ).

On the other hand, if you don't declare riichi and someone discard , you can't claim it since you have no yaku.

I'm not sure when it's worthwhile to wait also risking to lose a win because you don't have a yaku, and when instead is best to declare riichi.

I will give to you the same answer that was given to me in my early days on the forum.
It depends on the situation.
Yes, if you draw a or , your hand will improve, and you can riichi then.
Yes, if you don't riichi now, you risk losing a win if a is discarded.

In this case, it is EAST 1, and there is still lots of time to win the MATCH.
If a is discarded, you can still riichi later and wait for another one to come out.

What would you do if you drew a or . No more pinfu, but waiting for two different tiles. Do you consider that an improvement? Is riichi and a dora enough?

I don't think there is a general rule etched in stone on what to do.
In this case, I would probably wait for some improvement before I riichi.
Other considerations:
Am I dealer now? Is this a Hanchan? Everything matters.
(And I'm sure you know this, but calling riichi doesn't always guarantee a win, you can still toss that winning tile to someone else.)

It was hanchan, the player was north.