If knowledge is empirical, it's based on observation rather than theory. To do an empirical study of donut shops, you'll need to visit every one you can find.
Empirical looks like empire but comes from a completely different origin: it is from the Greek empeirikos, meaning "experienced." It was originally used in medicine for doctors making choices based on observation and experiment rather than theoretical ideas. It's now used for any kind of knowledge that comes from experience. You can meditate all day on the origins of donuts, but until you visit the donut bakery you'll lack empirical knowledge of donut creation.