Black Cat

A mathematician, a theoretical economist and an econometrician are asked to find a black cat (who doesn't really exist) in a closed room with the lights off:
The mathematician gets crazy trying to find a black cat that doesn't exist inside the darkened room and ends up in a psychiatric hospital.

The theoretical economist is unable to catch the black cat that doesn't exist inside the darkened room, but exits the room proudly proclaiming that he can construct a model to describe all his movements with extreme accuracy.

The econometrician walks securely into the darkened room, spend one hour looking for the black cat that doesn't exits and shouts from inside the room that he has it catched by the neck."