The problem is: yes, there's only a tiny chance somebody who searches 'nike shoes' ends up in your store. But the probability that someone ends up in your store BECAUSE he saw your ad is even smaller. So if you don't correct for selection effects your going to overstate ad effects by orders of magnitude.
The problem is: yes, there's only a tiny chance somebody who searches 'nike shoes' ends up in your store. But the probability that someone ends up in your store BECAUSE he saw your ad is even smaller. So if you don't correct for selection effects your going to overstate ad effects by orders of magnitude.