N=(R*)(Ns)(Fp)(Ne)(Fl)(Fi)(Fc)(L)
Where:
- N = Number of civilizations we can communicate with
- R* = Average star formation rate
- Ns = Percentage of stars capable of supporting earth-like planets
- Fp = Percentage of those stars with planets
- Ne = Average umber of planets in those stars' habitable zones
- Fl = Percentage of those planets where life begins
- Fi = Percentage of planets with life where an intelligent species evolves
- Fc = Percentage of those intelligent species that develop communication technology
- L = Average lifetime, in years, of a civilization that develops such technology
