I wanted to know how much a user uses scrollbar buttons rather than the thumb? and possible cases when he goes for buttons?
I have found a heat map study from mozilla ui lab for the firefox browser. It's visualize the user behavior clearly.
Based on over 117,000 Windows 7 and Vista Test Pilot submissions from 7 days in July 2010
The Up arrow on the scrollbar was used on average by 27% of users.
The Down arrow was used on average by 33% of users.
95% of users drag the vertical scrollbar