I'm a fan of the Drake as a thought experiment, but not any implementation I've ever seen. I don't mind the hand waving needed to keep the number of variables small enough for a single slide ( ;-) )   but that shouldn't (but always seems to) bias the Draker to select what I call "comfortable numbers"  (as in, numbers comfortable to the Drake user).  This is where Drake collapses, imo. Suppose we could agree that there are 10 (dependent) things that have to happen to make a planet habitable. My list of 10 might be slightly different than Lex's list of 10 or anyone else's. That's okay. No problem. BUT, the number assigned to the Drake variable for "inhabitable" is more accurately assigned to EACH of those 10 things, which of course drastically changes the actual Drake variable value that should be used.  "Intelligent life" may have 25 things, each getting the estimate the traditional Drake user would apply.  And so on.   One last use to make my point about comfortable number bias. Remember the way Jodi Foster's character laid out Drake?  "..if just one in a million of those... and just 1 in a million of those...  "  Why million? What's the valid argument that trillion isn't more accurate or quadrillion or, or...  other than "million" is  more comfortable than, for example, "nonillion."   If each of the 10 things to make a planet inhabitable is really a (dependent) 1 in a nonillion chance, the output of Drake is very different.  There seems to be nothing about the vastness of the universe that is congruent with the range of numbers comfortable to humans.