There are various hypotheses for why behavior functions may be difficult to identify. One explanation is that a single behavior serves multiple functions. The FACT is a 35-item measure that was constructed to identify a hierarchy of behavioral functions for individuals with Intellectual Disabilities who present maladaptive behaviors that have multiple functions, using a forced-choice procedure. Parents/caretakers familiar with the individual rates each item. Each item provides the informant with two choices with each choice representing a different function. These items are rotated so that each item has been paired with all of the available items. The frequency of endorsement indicates the overall validity of each item. This relative frequency of endorsements may indicate the treatment priority of the respective behavior functions. A sample item is, 'Engages in the behavior more when he/she does not want to do something, or more when there is something bothering him/her physically.'

References:

Matson, J.L., Kuhn, D.E., Dixon, D.R., Mayville, S.B., Laud, R.B., Cooper, C.L., Malone, C.J., Minshawi, N.F., Singh, A.N. Stewart, H.A., Luke, M.A. & Matson, M.L. (2003).The development and factor structure of the Functional Assessment for multiple CausaliTy (FACT). Research in Developmental Disabilities, 24, 485-495.