Sometimes, people who are unfamiliar with the scientific picture of cosmic and biological evolution think the equation is highly speculative. In fact, it is just the opposite, since each phenomenon it assumes to take place in the universe is an event that has already taken place at least one.(6)
The original formulation is: \[N = R_{\ast} \cdot f_p \cdot n_e \cdot f_{\ell} \cdot f_i \cdot f_c \cdot L\] where $N$ is the number of civilizations in our galaxy with which radio-communication might be possible (i.e. which are on our current past light cone);
$R_{\ast}$ the average rate of star formation in our galaxy;
$f_p$ the fraction of those stars that have planets;
$n_e$ the average number of planets that can potentially support life per star that has planets;
$f_{\ell}$ the fraction of planets that could support life that actually develop life at some point;
$f_i$ the fraction of planets with life that actually go on to develop intelligent life (civilizations);
$f_c$ the fraction of civilizations that develop a technology that releases detectable signs of their existence into space;
$L$ the length of time for which such civilizations release detectable signals into space(8)
In 1966 Shklovskii and Sagan added a new term to the equation, $f_g$, the fraction of stars suitable for supporting life(2, 3, 4).