NEW YORK — A study by Northeastern University professor David Choffnes tested thousands of Android apps and found no evidence that smartphones secretly record users' audio for advertising purposes. Choffnes set up a fake apartment equipped with smart appliances, cameras and smart speakers to study how much data the devices send.

"We did a study with thousands of apps on an Android device, and wanted to see, as you interact with these apps, are they recording your audio and sending it off?" Choffnes, a professor of Computer Science at Northeastern University, said. "We didn't see any surreptitious recording of information."

While the study found no covert audio surveillance, he said companies collect data through other means. "But these companies are very good at watching everything you're doing online," he said. As part of the research, he also examined what advertisers already know about users. "We try to identify, are they sending data to places we're not comfortable with?" he said.

Ad industry veteran, consultant and author Ari Paparo said the idea of phones listening for ad targeting is not technically feasible. "I can guarantee you that your phone is not passively listening to you for advertising purposes," Paparo said. "Listening to every conversation around the world, and interpreting them and looking for certain words, and then matching them to the ads is impossible."

Instead, advertisers rely on data collected from users' online behavior. "They can infer or deduce things about you, like where you live, and your age, and probably what you're interested in, based on what's websites you went to or what apps you've used," he said. He added that household internet usage can also create misleading ad targeting: "Your wife could have looked for a peeler, and then the ad company couldn't really tell the difference between her and you, because you're using the same internet in the same household."

Advertisers do not know individual users' names or addresses but do know the demographic categories users fit into. Choffnes obtained his own advertising data report, which was more than 300 pages long. The report incorrectly listed that he owns an Xbox and is extremely likely to go on a cruise. The study was conducted exclusively on Android devices; no iOS apps were tested.

Many states require data collection companies to provide consumers with their advertising profiles on request. "You can push your lawmakers to come up with laws that are favorable for consumers and not just favorable for the businesses that are collecting data from us," Choffnes said.