Where the variables represent: formula_2 is the number of technologically advanced civilizations in the Milky Way galaxy; formula_3 is the rate of formation of stars in the galaxy; formula_4 is the fraction of those stars with planetary systems; formula_5 is the number of planets, per solar system, with an environment suitable for organic life; formula_6 is the fraction of those suitable planets whereon organic life actually appears; formula_7 is the fraction of inhabitable planets whereon "intelligent" life actually appears; formula_8is the fraction of civilizations that reach the technological level whereby detectable signals may be dispatched; and formula_9 is the length of time that those civilizations dispatch their signals. The fundamental problem is that the last four terms (formula_10) are completely unknown, rendering statistical estimates impossible.