Clear text data!

The key questions are where are the data stored and who are you trying to protect them from?

If they are in internal storage, especially if they were internal app data, you'd be surprised how hard they are to restore. There is software you can buy that claims to do so, and most if not all of it is a con. Regular file recovery software won't work over an MTP connection (the protocol used when connecting to a computer via USB). So against regular civilians I don't think you have anything to worry about. Obviously a removable card can be removed and stuck in a card reader, so if you are worried about stuff on there then you need to be more thorough.

Now if you are worried about someone taking the phone and applying professional forensic tools then that's a different matter. To protect against that you'd want to write over the data as soon as they were deleted, which isn't how the OS works. Of course unless you are suspected of being a serious criminal and the authorities get their hands on your phone this is not at all a likely scenario, so I'd not expend too much effort worrying about it (indeed unless someone is going to have extended physical access to your phone I don't think you need to worry about any of this).

But if you are worried may I make a simple suggestion: encrypt your phone. That way even if someone were to get access to it and recover the data, the data are encrypted (and if they had to reset the phone to get past your screen lock then the encryption key has been wiped). That's the simplest way of making sure a phone is securely wiped before selling it, encrypt it and then do a factory reset, so this is the same principle.