Among the apps tested, the iPhone apps transmitted more data than the apps on phones using Google Inc.’s Android operating system. Because of the test’s size, it’s not known if the pattern holds among the hundreds of thousands of apps available.

Apps sharing the most information included TextPlus 4, a popular iPhone app for text messaging. It sent the phone’s unique ID number to eight ad companies and the phone’s zip code, along with the user’s age and gender, to two of them.

Both the Android and iPhone versions of Pandora, a popular music app, sent age, gender, location and phone identifiers to various ad networks. iPhone and Android versions of a game called Paper Toss—players try to throw paper wads into a trash can—each sent the phone’s ID number to at least five ad companies. Grindr, an iPhone app for meeting gay men, sent gender, location and phone ID to three ad companies…

Smartphone users are all but powerless to limit the tracking. With few exceptions, app users can’t “opt out” of phone tracking, as is possible, in limited form, on regular computers. On computers it is also possible to block or delete “cookies,” which are tiny tracking files. These techniques generally don’t work on cellphone apps.