# What is the two alternative forced choice paradigm

Is a two alternative forced choice paradigm (2AFC) an experimental design?

Two-alternative forced choice is a psychophysical method or paradigm used to solicit behavioral or perceptual responses from subjects.

The name comes from two aspects of the method. It is called two-alternative because the subjects are shown two alternatives (A or B), and forced choice because they are "forced" to choose one or the other, based on some previously-indicated criteria (e.g., brightness, moving rightward, etc); the subject isn't offered a way to avoid answering or choose "none of the above."

The term is sometimes used more loosely to indicate a task where the subject views a single stimulus and must make one of two responses. However, this isn't strictly correct and a lot of the signal detection results about 2AFC paradigms no longer hold in these cases. For example, Macmillin and Creelman (2005) (pg 166) write:

"In using the traditional name for this design, we continue an unfortunate historical precent. The choices made by observer in two-alternative forced choice (2AFC) studies are no more constrained than in other correspondence experiments. As in the one-interval design, the possible stimuli come from one of two categories (Old or New, Loud or Soft) and the experimenter is interested in the correspondence between the correct response and the observer's 'forced choice.' The new feature of the 2AFC design is that both alternatives are presented on every trial in random spatial or temporal order. The observer reports not which stimulus occured--both did--but in which order."

Most other sources seem to agree with this definition: Hautus, 2015; Schneider and Parker, 2013.

This distinction is important because a 2AFC task is much easier than making judgments about the individual stimuli in isolation. It can be shown that an optimal strategy does a factor of $\sqrt{2}$ better on a 2AFC task than when making jugements about the stimuli in isolation.

• Omg Thank you @Jeff and Matt Krause for taking the time to make sure I understand 2AFC, enormous appreciation :)! – Tina Mar 4 '16 at 21:33

Two-alternative forced choice (2AFC) refers to the methodology used to solicit responses from participants. It is used in many different paradigms across different domains. As the name implies, participants are presented only two choices, and they must make a response (indifference is not allowed).

For example, the lexical decision task is a 2AFC task. Participants are presented a sequence of letters such as BOKE and must quickly indicate whether the sequence of letters forms a word or a non-word.

Another example from psychophysics might ask participants to judge whether a test stimulus is brighter than a target stimulus. Participants must response either yes or no.

• I'm not sure the the lexical decision task "counts" as 2AFC. It's true that there are exactly two responses and the subject is forced to choose one of them, but there's only one stimulus, so it's effectively a yes/no task. You could turn it into a 2AFC task by showing one word and one nonword each time and asking the subject to choose the word. This sounds like a piddling distinction, but there are surprising differences in performance between the two paradigms because (e.g.,) you can use item A's "wordiness" or B's "non-wordiness" to guide your responses in a 2AFC task. – Matt Krause Mar 1 '16 at 22:48
• I agree that your suggestion could result in different behavior, but it isn't necessary to have two stimuli to be considered a 2AFC task. A stereotypical 2AFC task is the random dot motion task shown on the Wikipedia page. There is only one stimulus and the participant responds left or right. Which is not much different than a lexical decision task, in which participants response "word" or "non-word". Moreover, the computational models for 2AFC tasks on the wiki can (and have) been applied to the lexical decision task. – Jeff Mar 1 '16 at 23:01
• I'm not sure I agree. Signal detection theory draws pretty strong distinctions between yes/no tasks (lexical decision), "reminder" tasks where one of the stimuli is fixed (your third task), and 2AFC. This does mean it's possible to have task with two intervals and two forced choices which is not a 2AFC task, which is admittedly weird. – Matt Krause Mar 2 '16 at 8:24
• I agree with @MattKrause and think both Jeff and the wikipedia page are in error. – Jake Westfall Oct 7 '16 at 16:20
• There may be differences in the meaning of 2AFC by research area. In my experience with the DDM/LBA (etc) literature, "2 alternative" refers to response alternatives, hence the random dot motion task is a 2AFC task. I don't doubt that presenting two stimuli vs. one is psychophysically different, but that can be modeled in the DDM. – Jeff Oct 12 '16 at 17:41