The Register has a sobering piece today that examines how free Android apps pose a substantially higher threat of invading users’ privacy than premium apps do. Free apps from Google Play are five times more likely to track user location than paid apps, according to fresh data from Juniper Networks, and more than three times as likely to access users’ address books.

Juniper’s study was impressively extensive: The firm spent 18 months running an audit of 1.7 million apps in Google Play. Almost one-fourth of free apps requested permission to track location compared to 6 percent of paid apps, and roughly 7 percent asked for permission to access contacts information compared to 2 percent for paid apps.

Some apps requested permission to secretly make outgoing calls, send text messages or access the camera function. Perhaps most interesting, though, is that only 5 percent of free Android apps share that information with major ad networks like AdMob, Millennial Media and AdWhirl. “That leads us to believe there are several apps collecting information for reasons less apparent than advertising,” Juniper Networks concludes.

The study highlights the need for app developers and publishers to do more than just ask users for permission to access certain data and functions; they should also tell users how they plan to use or share that information. That’s especially true in Google Play, where free apps are more popular than in Apple’s App Store, and where apps are vetted less stringently than in other stores. And it’s particularly timely in light of yesterday’s news that California is set to begin fining app developers who don’t offer clear and easily accessible privacy policies.