Elaborations
As many observers have pointed out, the Drake equation is a very simple model that does not include potentially relevant parameters. David Brin states:
merely speaks of the number of sites at which ETIs spontaneously arise. It says nothing directly about the contact cross-section between an ETIS and contemporary human society.
Because it is the contact cross-section that is of interest to the SETI community, many additional factors and modifications of the Drake equation have been proposed. These include the number of times a civilization might re-appear on the same planet, the number of nearby stars that might be colonized and form sites of their own, and other factors.
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