A University of Chicago business school study shows that the lowly banner may be the most effective ad in helping retain customers, even better than email or pop-ups. It seems that customers find the banners much less intrusive than the other formats, and as a result react more efficiently to them for additional purchases. Non-customers, on the other hand, tend to be left cold by banners as a first means of communication.

The study also found that measuring banners by click, or even by just the initial transaction, significantly undercounted the ads' effectiveness. Accurate efficiency measurement happened only when users were tracked over time and multiple purchases, where banners had additional effects beyond the mere measure of the average lifetime value of the orginally-acquired customer.