The interviewer calls in the mathematician and asks: "What does two plus two equal?" 

The mathematician replies: "Four." 

The interviewer asks: "Four, exactly?" 

The mathematician looks at the interviewer incredulously and says: "Yes, four, exactly." 

Then the interviewer calls in the accountant and asks the same question. 

"What does two plus two equal?" 

The accountant says: "On average, four - give or take ten percent, but on average, four." 

Then the interviewer calls in the economist and poses the same question. 

"What does two plus two equal?" 

The economist gets up, locks the door, closes the shade, leans close to the interviewer and whispers: "What do you want it to equal?"