Drake equation
Although written as an equation, Drake's formulation is not particularly useful for getting a value of .  The last four parameters,  and  are not known. They are very hard to guess, with values ranging over many orders of magnitude. Therefore, the SETI League says the importance of the Drake equation is not in solving it, but in thinking about it. It may be more useful to think of it as a series of questions framed as a numbers game.