And then if you multiple it by the probability of the evolution of sentience, you get an estimate about the number of alien civilizations. If we only knew that either...
As I understand it [and someone correct me please if it's wrong] one of the most unknown variables is how long average civilizations last?
Since the only sentient species on this planet has been around ~ 20,000 years and we have know idea how long we will last or if we'll kill ourselves - or become zombies
- it's also as probable that many alien species existed for 50,000 years and then died off for a number of different reasons.
Technological advancement does not necessarily equate to immortality for a species.
The universe is billions of years old. Many aliens may have predated us and then died off long ago.