The button is on the bottom right. Through my statistics I have seen, that more than every third person clicks on that at some point during the survey. A few of them then start over, most of them don't.

My guess is, that most of these clicks are accidental as users see a button to the right and click without reading because that's the expected position for next question group button.

I have now moved this button to the left and will write about my experiences with that later.

7 December UPDATE: Placement was in fact wrong!

After I moved the button its click-rate is now down to one in ten users (not one in three as before) - the ones that just don't want to take the survey. Before replacement some started over again - even two or three times - a sign of accidental hits. Now only very few start over again after hitting the button - and only once at most.