It is not prejudice. It is common sense. If you use a major metro area with a large minority population, it will skew the results towards the idea that gun owners are more likely to be killed. If you were to do that same study in a rural town with a mostly white population the numbers would probably skew the other way. The only way to get a true conclusion would be to draw from a sample that includes the country as a whole with a number of cities that encompass all locales large to small with diverse population breakdowns.