The ad networks have behavioral advertising data (browsing histories) linked to cookies. Data aggregators have interest and purchase data linked to your PII. If existing customers have self-identified on a business' website, Web publishers and advertising networks can match up both data sets to predict more accurately who is most likely to respond to an ad.
But combining data from offline and online resources to deliver targeted advertising requires an elaborate dance, called cookie syncing, to ensure that a third-party advertising network does not receive any PII, says Barrett Glasgow. First the publisher sends the data aggregator, such as Acxiom, the PII data for its registered customers so it can be matched with the aggregator's profile data.
Acxiom then places cookie on the user's computer and gives a code to the ad network, which uses it to read the Acxiom cookie and pull the relevant demographic and interest data associated with it. It then uses both data sets to determine the most appropriate ad to send to the user. "In the online space there's this whole added dimension of complexity around anonymity," Barrett Glasgow says.
— Robert L. Mitchell
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