Each of them are brought into the bosses office individually and asked only one question.
The mathematician comes in and is asked "what is 2 + 2?" To which he answers "obviously it's 4"
The engineer is asked the same question. "Well it's hard to say for sure, I'd have to make a few calculations, draw up some graphs. Let me get back to you in about a week."
Finally, the statistician is brought in.
"What's 2 + 2?"
The statistician then casually closes all the blinds in the room and slyly asks "what do you want it to be?"