Two alternative forced choice

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2AFC is a abbreviation for Two Alternative Forced Choice.^[1] It is a psychophysical method for eliciting responses from a person about his or her experiences of a stimulus. For example, a researcher might want to measure the sensitivity of a person to dim light. On any trial, a dim light might be presented at random in the top part of a screen or in the bottom part. The person then has to choose whether the light appeared in the top or bottom. The person is allowed only to choose top or bottom; he or she is not allowed to say "Not sure", or "I don't know", or "I did not see any light!". This is the sense in which the person's choice is forced.