How is Knowledge v3 "Most useful" calculated?

The "Featured content", "Most useful" and "Most viewed" sections in Knowledge v3 are great. I'm wondering quite how the "Most useful" section is evaluated - seems to be related to clicks on the "Helpful? Yes/No" buttons, but it would be handy to know quite what is being calculated.

It doesn't seem in my limited testing to be just based on the articles with the highest number of "Useful" ratings. Is it based on number of "Useful" minus number of "Not useful" ratings? Or something more complex again? I've not managed to track down the code that generates this data.