North Carolina State University study Android platform and collect 100,000 apps from the official Android Market in March-May, 2011. They identify the possible 52.1% apps using Advertisements and further develop a system called AdRisk to systematically identify potential risks. Most of the ad libraries collect private information, some of them may be used for legitimate targeting purposes, while others are hard to justify by invasively collecting the user’s call logs, phone number, browserbookmarks, or even list of installed apps on the phone.
Source: https://thehackernews.com/2012/03/security-holes-in-android-with-apps.html