The assessor is presented with two or more ports, of which one presents the stimulus and the other(s) neutral gas.The location of the stimulus in consecutive presentation is randomly distributed over the two or more ports. The assessor is asked to indicate which of ports is the one with the stimulus. When the assessor doubt, he/she is asked to indicate a port ” at random”. To reduce variability and achieve convergence with the yes/no choice mode, the following procedure is applied to distinguish False from True responses. The assessor is asked whether his/her choice was a guess, an inkling or certain. From the combination of the choice result and the indicated level of certainty the response is classified to be False or True.