Forced-choice guessing floors push threshold criterion above 50 percent detection

Evidence

In some cases the value of the psychophysical function does not approach 0% at the lower end of the scale. This happens, for example, if the difference threshold is measured with the 2AFC method, where the chance of guessing correctly is 50% when the difference between the signal and the reference is below the threshold. In this case, the value of the threshold may be selected to be 71% or 75% of the maximum of the function. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 914

Categorization

• DEFINE: Explains why threshold criteria sit at 71-75 percent rather than 50 percent in forced-choice tasks.