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anyway. so what percentage of the time do you think is this a bluff?

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I think I'm getting bluffed much less than 10% of the time on the river. If this happens on the turn, then I think 10% or even a bit more is right. You've mentioned that the player is a bit loose but at least semi-thinking. If I suspect a player like this is making a move, I immediately think about how I've played the last few orbits to find any of my behavior which makes me look susceptible.

If I've ended up betting and missing some flush draws and he's seen that, then maybe I give a 10% chance of a river bluff. I guess roughly the same if he's seen me bet every street with a whiffed AK (which I don't do very often.). If I haven't done any of these things lately, I put the bluff percentage closer to 5% or less.

So here, I call when I'm pretty sure I'm beat for the following reasons:
1) If I fold, the likelihood of this happening to me again goes way up.
2) If he's not bluffing, I want to know why he took this line against me. It also doesn't cost me anything since I would have gone into check/call mode if he had popped this on the turn instead.