See the original Nature paper from 1959 that started Modern SETI
http://www.coseti.org/morris_0.htm
"The probability of success is difficult to estimate; but if we never search, the chance of success is zero."
Space/Science » in reply to Detailed Obituary
http://www.coseti.org/morris_0.htm
"The probability of success is difficult to estimate; but if we never search, the chance of success is zero."