I recommend using the free program Eraser to permanently delete your data. Eraser writes over files with meaningless data, making the files unrecoverable.

Permanently erase data from your computer Q. I'm giving away my old computer to my nephew, but I don't want him to see any of my files. I can just delete them from the Recycle Bin, right? A. Wrong!

The contents of your PC’s Recycle Bin or Mac’s Trash Can are only cleared out when you empty them. Kim Komando offers tips to permanently delete data.

Simply restoring the operating system to factory settings does not delete all data and neither does formatting the hard drive before reinstalling the OS.