Looks like an early version of the Drake equation, only with the number of stars with planets very low and the probability that a planet has life very (like really, really) high.
Overall, very intriguing. Adding your mention of the Drake equation got me looking into it a bit, added some flavor to this article. Thank you.
It is good that it gets us thinking about the potential for life and drives us to explore, but I suspect some of the calculations made at the SETI institute and such will be completely wrong.