There are two factors; how much and how likely. An example is when a company soon will come with an announcement on a court rule, and all you know is that it will change the value of your stocks with $5,000 or $-1,000. If you don't know the probabilities of these outcomes you can't tell whether the stocks are over- or under-priced.

The formula of the expected outcome is
E(X) = ∑xp(x)
or in plain English it's simply taking an outcome (x) multiplied with the probability of that particular outcome (p(x)), and do this with all potential outcomes and then adding all the products together (the sum of all the probabilities should be 100%). This formula isn't worth more than the accuracy of your estimates, but at least it gives a bit more insight than the traditional risk/reward comparison.

However, if you train yourself to use this formula mentally it might give you an edge in comparing different outcomes. The way I do is to split into a few boxes, with different outcomes on the horizontal axis and the probability of every outcome on the vertical axis. When the outcome doubles, the width of the column doubles as well. To do it more accurately, use excel or paper.