Scenario 8 – Sensitivity to Disfigurement

As part of your dissertation investigating our ability to detect facial disfigurement, you conducted a five-alternative forced-choice experiment utilising a single staircase approach. Faces were digitally manipulated with various degrees of disfigurement, ranging from very mild to extreme. Five images were presented on the screen, four with no disfigurement and one with, and the participants had to locate the face with the disfigurement. The proportion of correct identifications are presented below for one participant.

Disfigurement index (1=mild, 8=extreme)	1	2	3	4	5	6	7	8
% correct identification	0.23	0.28	0.37	0.49	0.75	0.92	1.0	1.0

Plot a psychometric function of the results.

What is the guessing rate in this experiment?

Estimate the threshold of disfigurement detection.

Some studies have shown that 5AFC studies are too cognitively demanding and that 3AFC approaches are more appropriate for better participant cooperation. What would the threshold have been if this was a 3AFC design?

Can you identify any problems with the experimental approach as described here?