This is a very legitimate concern. ReTargeting does add value and we do track click-through and view-through conversions, allowing advertisers to attribute sales appropriately. Additionally, comparing historic conversion data with ReTargeted conversion data will give you a bit more insight into the actual magnitude of the campaign. Lastly, comScore recently put out a study showing that retargeting adds the biggest lift in a brand’s search, moreso than any other type of ad buy. With that said, it’s important to compare direct traffic to your site during a retargeting campaign with the direct traffic from before a retargeting campaign.