Well there is the drake equation, which tries to simplify the idea of how many civilisations might be out there, but the problem is we have to guess at some of the values to begin with and since we don't have any contact with ANY thing out there yet, its kind of hard to prove the equation... and even worse, a number of wildly different answers are still right using this equation.
http://www.airynothing.com/smackerel...eEquation.html
I tend to think that any sentient species is worthy of contact by another, regardless of how primitive they are. Have we not come far enough now to be able to speculate about future technology to at least have a go of understanding it..
I think it more likely that advanced species may in fact protect those less advanced civilisations (like ours) for fear they could easily be wiped out by more advanced ones, or worse, be enslaved by them. Also imagine what we might do with a little of their technology.. we are after all no less warlike than the ancient romans. I think we need to get to a certain level of cultural maturity before they would consider contacting us, rather than think of us as ants..
Of course a few crashed ufo's would be handy to help us along
