How likely is it someone can recover data after a factory reset?

[Jason Cockerham]

Hey guys. So I'm wanting to trade in an old phone, but since I started a new job, I've had to have some sensitive data (documents, pictures and things) on the phone. I've already done a factory reset a couple times on the phone and my concern is that if I trade it in, then maybe the carrier (T-Mobile) or the next person who has the phone could still recover some of the data on the phone if they're good enough. My question is, what is the likelihood of that happening? How difficult is it to recover data after a factory reset? Is there any way to make it more secure without rooting it? I know the very basics of rooting but am not that tech savvy beyond that. The phone is a Note 4. I bought it new.

Thanks!