From the newsservice Omni:

Swedish people are bad at earsing personal data on their pc:s when selling them. Dagens Nyheter gave a security company this task: Buy used computers from Blocket, Tradera and second hand stores. When analysing it turned out that 42 out of 51 contained personal data. 39 contained large amount of data and 9 of then contained photos of credit cards or dokument with bankaccount numbers.
- I had expected we would find some data here we really found evrything. This was way more than expected said Calle Svensson, IT-forensics ar securitycompany Bitsec.

Sources in swedish: http://tt.omni.se/a115c0e41f603afa942b8d9cc19544041ea7ab2d
http://omni.se/sok?q=svenskar%20d%C3%A5liga%20p%C3%A5%20att%20radera&tab=

To really erase your mac do like this: On a newer mac, reboot and hold down cmd and r. The mac will now start i recoverymode. When started choose disk utility. Markera in left collum the unit you want t erase, if you are selling it choose the whole HD (usually top choice). Click on erase, choose name on hd, use OS X Extendes journaled ans Apple partion map.
Varning: once you click on the erase button you cant go back. This goes for system 10.11 El Capitan. There are even safr ways to erase data but if your mac have a ssd (solid state drve) this erase will make it very difficult to find any data.