"Why ET Hasn't Called," by Michael Shermer. Scientific American, August 2002, page33.
A formula proposed by astronomer Frank Drake in 1961 involves themultiplication of seven constants to estimate the number of technologicalcivilizations in our galaxy. With the possible exception of one of theconstants, the value of each constant -- for example, the fraction of planetswith life --is mostly speculation. In his column "Skeptic," Shermer attempts toestimate one of the constants: L, the lifetime of communicating civilizations.He writes that the value of L may be one that we can determine with someaccuracy by using data about civilizations on our planet. Using thathistorical data, Shermer estimates L to be far smaller than other publishedvalues and shows that we should expect no more than four technologicalcivilizations in our galaxy. Carl Sagan's estimate was one million.--- Mike Breen