A minor detail that I found interesting in the map from the previous post on Starbucks and McDonald's, is that I did not know that KFC and Taco Bell had higher revenues than Starbucks. I expected Starbucks to make more money than either because I know a lot more people that spend a lot of money at Starbucks as opposed to KFC or Taco Bell.
The problem with my logic is an idea called selection bias. This is where you come to the wrong conclusion due to the fact that the sample of people you know or study does not represent the population as a whole. So just because I don't know many people who eat at KFC or Taco Bell, that might just mean that the type of people I know don't eat there as much as the general population.
Another example of this bias is how students at Walker are surprised to find out that only 60% or so of high school students go to college right out of high school. There is a selection bias at work here since almost everyone you know at Walker or other Cobb county schools plan on going to college, but that does not represent the country as a whole. If you went to an inner city public school or lived in a very rural area, that percentage may seem high instead.
Can you think of any other cases where you had a misconception due to selection bias? Or perhaps a common selection bias amongst Walker students or people you know?