As specified by Cooper, Heron & Heward, (2007) pp. 65-66.
A function-based definition designates responses as members of the targeted response class solely by their common effect on the environment. For example, Irvin, Thompson, Turner, and Williams (1998) define hand mouthing as any behavior that resulted in “contact of the fingers, hand or wrist with the mouth, lips or tongue” (p.377). Figure 3.6 shows several examples of function based definitions.
Applied behavior analysts should use function-based definitions of target behaviors whenever possible for the following reasons:
- A function-based definition encompasses all relevant forms of the response class. However, target behavior definitions based on a list of specific topographies might omit some relevant members of the response class and/or include irrelevant response topographies. For example, defining children’s offers to play with peers in terms of specific things the children say and do might omit responses to which peers respond with reciprocal play and/or include behaviors that peers reject.
- The outcome, or function, of behavior is most important. This holds true even for target behaviors for which form of aesthetics is central to their being valued as socially significant. For example, the flowing strokes of the calligrapher’s pen and the gymnast’s elegant moments during a floor routine are important (i.e. have been selected) because of their effects or function on others (e.g. praise from the calligraphy teacher, high scores from gymnastics judges).
- Functional definitions are often simpler and more concise than topography-based definitions, which leads to easier and more accurate and reliable measurement and sets the occasion for the consistent application of intervention. For example, in their study on skill execution by college football players, Ward and Carnes (2002) recorded a correct tackle according to the clear and simple definition, “if the offensive ball carrier was stopped” (p.3).
Function-based definitions can also be used in some situations in which the behavior analyst does not have direct and reliable access to the natural outcome of the target behavior, or cannot use the nsurl outcome of the target behavior for ethical or safety reasons. In such cases, a function-based definition by proxy can be considered.
For example, the natural outcome of elopement (i.e. running or walking away from a caregiver without consent) is a lost child. By defining elopement as “any movement away from the therapist more than 1.5m without permission” (p. 240), Tarbox, Wallace, and Williams (2003) were able to measure and treat this socially significant target behavior in a safe and meaningful manner.