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The number of such civilizations N is given by the product defined in the adjacent figure.
As a way to focus attention on the relevant factors, this equation is useful. As a useful equation it
is, well, not very useful. The first three factors we can perhaps assign with some confidence,
but the last three are at best guesses. Thus, the number that comes from taking the
product is at best a
guess. The answer could be one, or it could be billions, depending on what
assumptions are made.
Obviously, for this equation to give a
meaningful estimate, we must have data on other civilizations (see the right panel)!