If you don't believe in random sampling, next time you go to the doctor for a blood test, have him take it all.
Some basic answers follow.
Simply put, we report what people think, along with what they do, because it is important. The difference is in the selection of our sources. You get the point. It only takes a tiny drop of blood, randomly drawn from the body, to test for cholesterol. A single drop is still enough to complete the test.