Google is keeping an eye on what you’re buying offline in addition to monitoring your online shopping in its latest attempt to sell more digital advertising. The offline tracking of most credit and debit card transactions will help Google to automatically inform merchants when their digital ads translate into sales at a brick-and-mortar store. Google believes the data will show a cause-and-effect relationship between online ads and offline sales. If it works, it could help persuade merchants to increase their digital marketing budgets. The Mountain View, California, company already runs the world’s biggest online ad network, with $79 billion in revenue last year.