https://www.beatthegmat.com/post268159.html#268159
Here's an example of the kind of question that could appear within the "normal distribution" topic.
There's a lot of debate about this, varying from students claiming to have seen this on the actual test, and some prep companies that vehemently claim this is not in scope.
I am going to trust my test prep company - Knewton, which said that such questions are fair game on the GMAT, though very rare. Worth it to take a look just so you are familiar, since they are not very difficult to handle once you know the facts.
If, however, you have your doubts about whether something that isn't put forward in the OG is going to appear on your test, then I recommend you don't bother. Stick to the statistics questions covered in the OG.
