I never understood how exactly one figures out these three: f_l = the fraction of the above which actually go on to develop life f_i = the fraction of the abo

I never understood how exactly one figures out these three: f_l = the fraction of the above which actually go on to develop life f_i = the fraction of the above which actually go on to develop intelligent life f_c = the fraction of the above which are willing and able to communicate For f_l, we've got a sample size of one. For all we know, life as we know it might be a fluke which has an infinitesimal probability of occurring.