Numerous advertisers assess the quality of the traffic only by conversions to the ultimate event (purchase etc). This is wrong. In fact, the majority of purchases occur quite late into the campaign and user's lifetime, it takes time for a user to purchase, so you need to set up a conversion events funnel, register each conversion and crunch the numbers. For example, in case of a mobile game the events can be 1) getting through the Tutorial, 2) buffing up to level 10 and then 3) making the first payment. For a flight booking app the chain of events may include the first search, then the tap on a search result and then the payment.