Have you already heard about AIDA (Attention, Interest, Desire, Action)?
That's an advertising model I discovered a few years back (in 2010) when I purchased a tracking software called "Conversion Prophet". That software was really amazing, it's not working anymore and not being sold anymore, but you can still check its sales page on the way back machine here.
Basically AIDA allows to predict the traffic quality on a placement (keyword) basis extremely quickly, so fast that you don't have to spend too much money on any given placement before deciding what to cut and what to keep.
It does that by assigning time duration for the first 3 letters of the AIDA acronym:
For example, if a visitor spends 20 seconds on your page, the A(ttention) is triggered.
If he spends 2 minutes on your page, the I(nterest) is triggered.
If he spends 5 minutes on your page, the D(esire) is triggered.
And if he converts, the A(ction) is triggered.
When those durations above fit the content length of your lander or offer's sales page, you end up with a striking correlation between the number of recorded Desires and the number of Actions (conversions).
I'm using that model for my campaigns, and it helps to cut bad placements without paying an arm or a leg during the optimization phase.
If you'd like to learn more about AIDA, here's a case study showing how to use it in your affiliate marketing campaigns.