People are always confusing pre-judice with POST-judice. For instance, if you right now made some assertion about the Wanka-Ponga people of Southern Borneo, you would be acting in a prejudicial manner without sufficient information, such as the fact that the Wanka-Ponga people don't exist because I just made them up. However, what most people call "prejudice" simply involves having actually met members of others groups and observed the objective reality of them. Again, POST-judice, which is another way of saying you possess the ability to observe reality and draw conclusions, which is another way of saying you are not a sponge.
I'll add just one more point myself: Post-judice can also come about not only from one's own personal observation, but from the trusted testimony of others who have proved reliable in the past. For example, I've never met a Chechen, but my opinions about them are formed from what I've read about them and learned from trustworthy observers.
Quibcag: Somehow, an illustration of Akane and P-chan from Ranma ½ (らんま½) seemed appropriate. I really can't say why.