How YouTube’s advertising platform works
For each advertising slot—someone watching a YouTube video—Google runs an instantaneous auction, bidding on behalf of each advertiser, to determine which advertiser’s content will be shown (like search or display ads). YouTube’s advertising platform (Google Ads) allowed us to select a target population (our treatment counties) and to specify our willingness to pay for 1000 ad impressions (CPM), known as a “mille” in advertising lingo. We use CPM to gauge the cost effectiveness of our campaign. An advertiser is more likely to win an auction if she is willing to pay more than other advertisers and if Google predicts that, for a given user, the advertiser’s content is more likely to generate user engagement (which Google defines as watching at least 10 s of the ad). As we describe below, these features generated widely varying exposure to our ad across different counties, as Google’s algorithm dynamically adjusted as it learned which types of users were more likely to engage with the ad.