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Putting these factors into the Drake equation we get   $N \approx \, \frac{1}{4}\, f_i \, L $.   This states that the number of communicative civilizations in our Galaxy is just the product of the chance of emergence of intelligent life times the average lifetime of a civilization's communicative phase. In the next and last post of this series I will show how the current search for radio signals from extraterrestrial intelligence has put some important constrains on the two factors $f_i$ and $L$, and how these informations can be used to make some predictions about the future of humankind.  %Note this is basically the same result that Frank Drake obtained in 1961! The difference is that this has been %obtained only making assumptions on the factors $f_i$,$f_l$ and $f_c$, since the other terms in the equation %are now established.