In a report from Bloomberg, the publication has exposed an alleged deal that Google and Mastercard have struck up, in which basically in order to see the effectiveness of online ads, Mastercard would supply Google with purchase information. How it works is that when a user browses a web using their Google account, Google will log the ads that they come across and click.
However if an offline purchase is made in the next 30 days using a Mastercard credit/debit card, Google will then alert the advertiser via a report about the effectiveness of the ad, and will also include an “offline revenue” section that will take that sale in account. This deal with Mastercard is said to have cost Google millions, although neither company has officially announced or confirmed this partnership.
That being said, Google has recently come under fire recently for its location tracking feature, in which they did not make it clear that users had to do more than just disable location tracking if they want to opt out of it completely. In the meantime companies such as Apple and Mozilla have plans to block ad trackers by default, where they will prevent these trackers from tracking users across multiple websites.