Even on the river you can use math to help you. For example, if you think that the hand that beats you makes up only 25% of the guy's range given the betting, you could decide that in this situation you win 75% and lose 25% of the time with a call. You can still figure out odds to call. If villain makes a huge bet on the river,however, the math sort of blows up and you have to follow your gut. Once again, even if the math of the call makes it correct, you can use your reads, the stack sizes, stage of tournament, betting patterns, and gut feelings to swing the pendulum towards the correct decision.