For some time now Web survey designers have recognized that drop-downs are problematic, and so most avoid them. Web usaiblity guru Jakob Neilsen, who has long railed against them, has once again spoken on the issue. Below is his verbatim post:
Another reason to use drop-down menus sparingly, as I wrote in 2000:
Most users now have mice with scroll wheels. When you select a value in a drop-down and then use the wheel to move down the page, you often change the value of the drop-down selection instead. Users often don't realize that they have changed their input but proceed to submit the form.