Ad click prediction: what are the negative examples? I am analysing the log of a website and I would like to build a classifier to predict the users that are likely to click on an Ad.

The Ad can be displayed to the visitor several times.

To build any classifier I need positive and negative examples:


The positives are the visitors who clicked on the Ad (easy).
The negatives are the visitors who saw the Ad but didn't click (not very obvious).


Question


Is there a convention about how/when to consider a user as a negative example?


I presume that I should define a threshold of impressions (views) per user, if the user reaches this threshold without clicking on the Ad, I consider him/her as as negative example?

Any reference or guidance is highly appreciated.
You're overthinking it.  You might not need a threshold.  Start with the simplest approach you possibly can: If you showed the ad to a visitor, then that's a negative example.  Each time you show an ad to a visitor, you end up with an instance, whether positive or negative.  If you've showed the ad three times to the same visitor, you end up with three examples.  (Maybe all negative; or maybe one is positive and two are negative; but that's fine.)

Bonus tip: Do research methods for handling class imbalance.