Why professors shouldn't curve grades according to a normal distribution.

 A normal distribution is where scores of some kind are distributed like this. 

Now, I've heard stories of college professors at prestigious colleges grading their classes according to this distribution. However, that is a mistake; they are falling victim to survivorship bias. Back during World War Two, the air force needed to know how to armor their planes better. Because the planes could only take so much weight, they needed to know where to put the armor. They took a look at planes coming back from the battlefield to see where they got hit the most. They thought that they should put the armor where the bullet holes were. 

However, they were thinking about it backwards. Only the planes that weren't shot down returned. Thus, they should have placed armor where there were no bullet holes on returning craft as opposed to more bullet holes. Thankfully, they realized this and we won the war. 

The only people who are going to be in a high level class at a high level college have already "survived" to the point where, at very prestigious colleges, only the top two percent or so of applicants got in. Thus, professors should see a grade skew high. It makes no sense to grade the best minds in the world on a complete normal distribution chart. Any professor who does this is falling victim to survivorship bias. 

I've never met a professor who did this in person. I hope no one actually uses this method of grading. I'll rest my point here.