We do constrain the Drake equation when we know more. It's just that we don't know a lot. We have come to know the number of stars around pretty well - at leas

We do constrain the Drake equation when we know more. It's just that we don't know a lot. We have come to know the number of stars around pretty well - at least in our galaxy. Thanks to Kepler and other exoplanet hunters, we know know alot more about the number of planets. These findings are used to constrain the Drake equation. The rest of the parameters we don't know much about, yet.