"The problem with the Drake equation is that only f_p—and to a lesser degree n_e— is subject to experimental determination... [O]ne must have a fairly large sample; for f_l, f_i, and f_c we have only... the Earth. However, if... any intelligent species... will begin... galactic exploration within 100 years after developing... interstellar communication... the sample size is enlarged... Since f_p—and n_e can... be determined by direct astrophysical measurement, the fact that extraterrestrial intelligent beings are not present in our solar system permits us to obtain a direct astrophysical measurement of an upper bound to... f_lf_if_c, which depends only on biological and sociological factors."