The Drake Equation can be used to estimate the number of communicating civilizations in the milky way galaxy. First proposed by astronomer Frank Drake in 1961, the equation uses a series of variables to calculate the number of communicating civilizations, N. The variables each represent some cosmic parameter (mostly fractions) that influences the likelihood of existing civilizations (see below).
While some of these parameters are pretty well defined, many are up to the discretion and imagination of the scientist. For example, the number entered for the variable L can make a huge difference in the final number. An optimist who might believe that humans will last for millions of years will input a much, much larger number than someone who enters a number reflecting that the human race is past the halfway point. Either number is a fine choice–it is pure speculation. Therefore this equation, although interesting, cannot be taken as an accurate predictor for possible life within our galaxy.