Is data really deleted?

I just read this article about how things never get eliminated completely of your hard disk unless you destroy de hard drive. 

The article talked mostly about this experiment about some students made of buying computers and searching for private SUPER IMPORTANT information (such as credit cards, fotos, and important passwords and data) and got like 74% of that information back, and only 9% of people had correctly deleted everything on their computer.

I talked to my mother about this and she wasn't completely shocked because she says that one of her friends is really good with technology and that she's always telling those things to her friends. But when I talked to my father about it he REALLY was shocked because he truly didn't know that the things he erased weren't completely gone, and because he got one of his computer stolen he was super nervous about the information that he had that. 

Now I tell you what would you do if all the information that you've ever written, seen or played was founded by someone else?