• by Wendy Davis , Staff Writer @wendyndavis, November 16, 2015

Ad networks, publishers and other companies that collect data from someone's laptop in order to serve ads to that same person's smartphone must notify people about the practice.

That's according to the self-regulatory group Digital Advertising Alliance, which Monday unveiled guidance regarding cross-device tracking. The "Application of the DAA Principles of Transparency and Control to Data Used Across Devices" aims to tell ad networks (and other companies) how to comply with the industry's privacy code when tracking people across more than one gadget.

Companies engaged in behavioral targeting also must refrain from using data collected on one device to serve ads on a different device when people opt out.

In practice, this restriction means that if a user opts out on a laptop, marketers can't use data collected from that laptop to serve ads on any device linked to the person -- including smartphones and tablets associated with that user.

Even with the new principles, people who want to completely avoid targeted ads still must opt out on each device individually.

The new guidance builds on the DAA's longstanding privacy rules, which require ad networks and other companies to notify people about online behavioral targeting and let them opt out of receiving behaviorally targeted ads. (The rules don't require companies to stop collecting data about people who have opted out.)

When the industry developed its privacy code, most online behavioral targeting took place on desktops and laptops, but not smartphones or other devices. In the last several years, however, companies have increasingly sought to track people across smartphones, tablets and an assortment of other devices, and then serve ads to people based on their Web-surfing activity across the gamut of devices. One result is that now, when people visit an ecommerce site on their laptop, they might later see targeted ads from that retailer on their smartphones.

The DAA's new guidance was mentioned this morning at the Federal Trade Commission's workshop on cross-device tracking. Well-known privacy advocate Jonathan Mayer pointed out during a panel discussion that the industry could have developed a more universal approach -- such as a scenario where "exercising choice on one device percolates to all other devices."

Stu Ingis, counsel to the Digital Advertising Alliance, tells MediaPost that the organization is considering that concept, but says "there is complexity in the technological feasibility of such an approach."

Ingis adds: "Most consumers today only are using a couple of devices, so device by device choice for those who want to exercise it is not burdensome."