[MrB]

each factor is based on requirements for technological life on earth.

Take away the “factor” and tech life simply doesn’t happen on earth.

Then, the probabilities of each can be “guessed” at, but they certainly aren’t greater than the generous figure given of 10%. So, yes, the 1/100th of a chance per spiral galaxy is a WAG, but it is a WAG on the GENEROUS side.

[messierhunter]

Then, the probabilities of each can be “guessed” at, but they certainly aren’t greater than the generous figure given of 10%.

I don't know about that. 10% doesn't seem generous at all with regards to the percentage of stars with planets. Given the rate we're finding them with incredibly insensitive methods (which would probably return a negative result on our own solar system from a distant location), I'd say that first unknown ought to be at least 50%, if not higher. I'd also say 10% is pretty cynical with regards to the percentage of intelligent life that ends up emitting detectable radio signals at some point. For other variables, however, 10% could be generous.