An early (pre-interstellar age) attempt to estimate the number of civilizations in the galaxy, according to the statement that the fraction of stars harboring intelligent life equals the number of all stars times a sequence of fractions, such as the fraction of all stars having planets, the fraction of planets that are habitable, and so on.
Atomic age Old Earth radio astronomer Frank Drake was the first to formulate an equation which represents the probability of there being other civilisations in the galaxy capable of communicating. However, owing to the limited knowledge of the time, there was little solid data on which to base the numbers, and the answers vary widely and according to personal preference. New astronometric data and new theories greatly modified estimates of these numbers in one direction or another over the course of the Information Age.
The equation is :
N = Rs x Fp x Ne x Fl x Fi x Fc x L
N number of civilisations capable of communicating
Rs Rate of star formation (per year)
Fp Fraction of stars with planets
Ne Number of planets per star with suitable environment
Fl Fraction of planets on which life appears
Fi Fraction of life-bearing planets on which intelligence appears
Fc Fraction of intelligent societies which develop communication
L Longevity of society in communicative mode (years)