Frank Drake : Formula for extraterrestrial life

In 1961 put the U.S. astronomer Prof. Frank Drake in front of a formula with which he had calculated the number of communication-capable civilizations in the Milky-Way, roughly: N = R x P x E x L x I x Z x S. In the following example example: Formed in the Milky Way ten new stars per year (R = 10), half of all stars have planets (P = 0.5), of which two generally allow life (E = 2). Where life is possible, which is also produced (L = 1). Life leads in half the cases to an intelligent species (I = 0.5), but only one in ten is able to send signals into space (Z = 0.1).

Taking as their life span to 1 million years (S = 1,000,000), there should be alone in the galaxy N = 500,000 higher standing civilizations. Even after Drake's calculations, it would therefore - contrary to our perception - in the galaxy teeming with life that way. What can possibly be inferred: either Fermi's and Drake's calculations wrong, or the humanity the signs of extraterrestrial intelligence can not be identified.