There is a major factor that no one is mentioning here. When someone clicks your ad, sees your landing page, then hits the back button real quick, google will know this and your QS will take a big shit. Make sure your landing page holds their interest, matches what your ad says (especially the headline).
If you use tracking202, turn on 'show all clicks' (not only real clicks). Occasionally you'll see the click happy schmuck click your ad. Go to your LP. Click your offer link super fast (he didn't read shit) and then hit the back button all the way back to google. You'll see a red icon and his IP address again when he does this. Google will log this info as well.
This is most likely to happen with an ad that promises absolute bullshit that is irresistable and must be clicked. Of course they'll click, and of course they'll hit the back button when they confirm your ad was untrue.