Frank Drake, came up with the following equation to estimate the number of extra-terrestrial civilisations with which we might be able to communicate:

N = R_* × f_p × n_e × f_l × f_i × f_c ×L

Which can be read as the rate of formation of suitable stars * the fraction with planets * fraction which can support life * fraction where life develops * fraction where intelligence develops * lifetime of civilisations

Why is this one of the Things Which Everyone Should Know? Because it shows how you can increase your knowledge of a low information situation.

• Good short description - http://www.seti-inst.edu/science/drake-bg.html