Only a few years later, in 1961, the nebulous assumptions Cocconi and Morrison parlayed in their article got a bonafide mathematical equation. Frank Drake [with equation, at left], along with a handful of other astronomers and scientists (including Carl Sagan) met in Green Bank, West Virginia to hash out the formula and variables necessary to make an educated guess at just how many intelligent civilizations might be living in our galaxy. As it turns out, assigning numbers to nebulous assumptions nets you an answer with enough variance to make you wonder if you were really clarifying those assumptions in the first place. The group came up with a range from less than a thousand to nearly a billion.
You might think the formula would have been refined over the years, but that is not the case. It has held up surprisingly well (though, for such a nebulous equation "held up" is a relative phrase). Data collected since the 1960s, which can be used to support the original estimates of measurable quantities like how often sun-like stars form and how many of those stars have planets, has proven those estimates to have been relatively accurate. The rest of the variables will never be quantified, such as what fraction of life evolves to become intelligent and what the average lifetime of an intelligent civilization is. Still, the equation has served as a focal point for SETI investigations over the years and continues to be valuable framework, however controversial.