Want To Ensure Your Personal Android Data Is Truly Wiped? Just Turn On Encryption Then Reset
Avast's self-serving study was to promote its Anti-Theft app available on Google Play. The free app comes with a wipe feature that overwrites all files, thereby making them invisible to casual recovery methods, such as the many undelete utilities that are all over the web. That's one approach.
There's another solution that's incredibly easy and doesn't require downloading and installing anything. Before you sell your Android phone on eBay, Craigslist, or wherever, enable encryption and wait for it to encrypt its storage. After that, perform a wipe and reset as normal, which will obliterate the encryption key and ensure the data on your device can't be read.
Not all versions of Android support encryption, and if yours doesn't, you'll need to download an app that will do it for you. Assuming you own a relatively modern Android phone, you need to setup a lock screen PIN before you can proceed. You'll find that option by going to Settings > Security > Screen Lock > PIN.
Depending on your device, you may find the encryption option in the Security screen. On an HTC One, you need to navigate to Storage > Phone storage encryption. Before you proceed, you need to keep some things in mind. First, you can't reverse this process. The only way to go back is to perform a factory reset. Secondly, you may notice that your phone runs slightly slower. And finally, be sure that your phone is plugged in before you proceed -- encryption can take an hour or more.