The Drake Equation is used to estimate the possible number of planets in the Milky Way Galaxy with civilized life forms on them. Here is a cool video of Carl Sagan doing a great job explaining it:
http://www.youtube.com/watch?v=0Ztl8CG3SysBasically, the equation starts with the total number of stars in the Galaxy. From there you determine which ones have planets, and narrow down which planets are capable of sustaining life. You eliminate prospective planets by multiplying the total number of remaining planets by the fraction of planets that meet specific requirements for sustaining life.
For example, say there are 100 stars in a galaxy. 1/2 of the stars have planets (50 stars left). 1/2 of those planets are the proper distance from their sun in order to sustain life (25 stars). About 1/2 of those planets have the right mixture of elements in their atmosphere (about 12 stars). Only 1/2 of those planets have developed life capable of communication (6 planets). And so on and so on. I by no means understand what it takes to develop life on a planet but I think I have a basic grip on the equation. You start with a large number, apply specific criteria, narrowing the number down over and over again.
The other night I applied the Drake Equation to a more pertinent question: How many possible "loves" are there out there for one person?
You can substitute a variety of words for "loves": "soul mates," "suitors," "partners," etc...
What I did was start with the population of Iowa - Roughly 3,000,000
About 1/2 of those people are women - 1,500,000
I estimate that about 7% (about 1/14) of those women are in the age range of 20-27 - 105,000
About 1/2 have or will have a college degree - 52,500
I'll estimate that 1/4 of those women are single - 13,125
I'm attracted to 1/3 of those women - 4,375
I find 1/3 of those women's personalities pleasant- 1,458
I share common interests with 1/2 of those women - 729
Without going into any other factors yet we've estimated that about .02% of the population is suitable as a possible girlfriend. (more than 7 girls per county)
Now...
Let's say that 1/3 of our remaining population doesn’t mind looking at me too much - 243
1/3 find my personality interesting - 81
We now have estimated that I am compatible with about .002% of the population.
If you believe that we each have just one soul mate then the odds are even more daunting.
We're looking at 1/6,700,000,000 - Not quite bailout numbers but still pretty high.
It's amazing that people beat these odds everyday and fall in love.
THANKS