Researchers at the University of Cambridge conducted a study on Android devices from 5 different vendors and found that more than 500 Million Android devices don’t completely erase data after the factory reset. About 630 million devices may not completely wipe the internal SD cards where multimedia files such as pictures and video are kept. Researchers found that the data could be recovered from the Android device even if users turned on full-disk encryption. Google offers an option to protect your Google-related services such as Gmail, Maps and Maps.
Source: https://thehackernews.com/2015/05/android-factory-reset.html