Frank Drake, one of the early pioneers of SETI, devised a formula – the so-called Drake equation – to help assess the number of other races in the Galaxy who might be attempting to communicate with other stars at any given time. This formula, however, can produce a very high number or a number close to zero depending on the initial assumptions made.

 

Take Two
"Personally, I don't think there's intelligent life on other planets. Why should other planets be any different from this one?"    – Bob Monkhouse, English comedian

 

References

1. Trefil, James. Are We Unique? London: Wiley (1997).
2. Zuckerman, Ben, and Hart, Michael H., eds. Extraterrestrials: Where Are They? 2nd ed. Cambrdige, England: Cambridge University Press (1995).