Two-alternative forced choice

Two-alternative forced choice (2AFC) is a method for measuring the sensitivity of a <a href="/facts/Research_participant/6UvyWXn5">person</a> or animal to some particular sensory input, <a href="/facts/Stimulus_(psychology)/el6C43FY">stimulus</a>, through that observer's pattern of <a href="/facts/Choice/jCJIwQjm">choices</a> and response times to two versions of the sensory input. For example, to determine a person's sensitivity to dim light, the observer would be presented with a series of trials in which a dim light was randomly either in the top or bottom of the display. After each trial, the observer responds "top" or "bottom". The observer is not allowed to say "I do not know", or "I am not sure", or "I did not see anything". In that sense the observer's choice is forced between the two alternatives.
Both options can be presented concurrently (as in the above example) or sequentially in two intervals (also known as two-interval forced choice, 2IFC). For example, to determine sensitivity to a dim light in a two-interval forced choice procedure, an observer could be presented with series of trials comprising two sub-trials (intervals) in which the dim light is presented randomly in the first or the second interval. After each trial, the observer responds only "first" or "second".
The term 2AFC is sometimes used to describe a task in which an observer is presented with a single stimulus and must choose between one of two alternatives. For example in a <a href="/facts/Lexical_decision_task/pQG85TPu">lexical decision task</a> a participant observes a string of characters and must respond whether the string is a "word" or "non-word". Another example is the random dot kinetogram task, in which a participant must decide whether a group of moving dots are predominately moving "left" or "right". The results of these tasks, sometimes called yes-no tasks, are much more likely to be affected by various <a href="/facts/Response_bias/KNaQzWlL">response biases</a> than 2AFC tasks. For example, with extremely dim lights, a person might respond, completely truthfully, "no" (i.e., "I did not see any light") on every trial, whereas the results of a 2AFC task will show the person can reliably determine the location (top or bottom) of the same, extremely dim light.
2AFC is a method of <a href="/facts/Psychophysics/yDPmvvGf">psychophysics</a> developed by <a href="/facts/Gustav_Theodor_Fechner/egvj6XRA">Gustav Theodor Fechner</a>.

Two-alternative forced choice open-in-new

Two-alternative forced choice