I am going to "pile on" to all the great dialog here, mainly because I find this subject quite interesting.
Rule of 2 and 4: This is a good simple rule of thumb for calculating the PROBABILITY of hitting the CARDS we seek. If we will pay to see one card we can use 2x the number of outs to calculate the probability of hitting those cards. If we will pay to see two cards then we can use 4x the number of outs to calculate the probability of hitting those cards. This does NOT tell us our equity to win the hand. It is only the probability to improve our hand. Some of these cards are guaranteed winners and some are just better.
Pot odds and equity  the mathematical answer: If our equity to win at showdown is higher than the pot odds, then we can call a bet. Even if we have 34% equity with 33% pot odds. In my opinion, this is only OK in cash games because in the long run the 1% edge will play itself out. In tournaments, we need much more of an edge, because we cannot afford these kind of coin flips in every single hand we play. If we are going to split hairs with this small margin, we better be certain our equity is correct. I personally do not rely on this, especially early in the hand. We should be suspicious about a formula that may tell us to call with 72off.
Pot odds and equity  the practical answer: Is our hand really going to make it to showdown? If we will fold to pressure later in the hand, then we never really had the equity we thought we had. On the river, this is very clear, because there is no further action. But preflop or on the flop, there is a lot of poker to play. There are two concepts that describe this further  implied odds, and equity realization.
Equity realization: the idea behind equity realization is to ask ourselves "can we realize the equity we think we have". For example a 72off hand is just as likely to get a 3 of a kind as any other two unmatched cards  even AK. However, this probability applies only to the chances of accomplishing this with 7 cards. So, we may not see the 3rd 7 or the 3rd 2 until the river. Will we call the flop and the turn with a pair of 2's? This is equity realization. The mathematical equity is correct, but we will probably never see a showdown. So 72off has very little equity realization, so calling based on pot odds alone is sort of a disaster waiting to happen.
Hopefully this has shed some new light on this.
Good luck and God bless.
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