What percentage of visual stars are actually binary stars?

This is a difficult question, It's been changing through the years and it's difficult to calculate accurately. Now it's know that this percentage changes depending on the star type and ranges from 50% for Sun like stars up to a 80% for type O stars.

Fonts:

http://www.space.com/1995-astronomers-wrong-stars-single.html (2006)

http://www.space.com/22509-binary-stars.html (2013)