Count only if certain number of results returned

I'm a rookie here, and need some assistance.

So I've got a table of data where each instance an event happens, it is stored in the table with variable X (a measurement, not sure if that's relevant) and the name of a the person who did it. Events aren't grouped by the persons name, so a person can show up many times in the table. I'm trying to find the standard deviation of X, but I want to include only people who did "X" 10 times. Is there a way to do this? I was trying to use IF but didn't have too much luck.