The Drake Equation was developed by Frank Drake in 1961 as a way of determining the number of technologically advanced galaxies in the galaxy. The equation takes into account seven variables to develop a formula to quantify the odds of finding intelligent extraterrestrial life.
The first variable is the rate of star formation in the galaxy, which identifies the number of new stars formed in a year. The equation also considers the fraction of stars that form planets, the number of planets hospitable to life and the fraction of these planets on which life actually emerges. Another variable is used to determine what fraction of life-sustaining planets have intelligent life, whereas the fraction of planets with intelligent beings capable of interstellar communication is identified as another variable.
The last variable in the equation is the length of time an intelligent civilization remains detectable. According to How Stuff Works, this is the only variable astronomers feel most uncertain about. Some scientists estimate that civilizations remain detectable for 10 years while others estimate it to be 10 million years. Astronomers also use educated guesses about the rest of the variables to come up with a rough estimate on the total number of intelligent civilizations in the galaxy. When Frank Drake ran the numbers in the equation, he predicted that there might be 10,000 detectable civilizations in the Milky Way.