A functional analysis of the Wong-Baker Faces Pain Rating Scale: linearity, discrim-inability and amplitude

Abstract

Background: Self-report measures of pain intensity are often treated as interval level measures, which is a rarely tested assumption. Objectives: To assess the degree to which the Wong-Baker FACES Pain Rating Scale (FACES) provides interval properties in samples of children differing in age (6-8 and 9-11 years old) and pain experiences. Methodology: The study is based on the Functional Measurement methodology, which offers both an empirical criterion to validate the linearity of response scales and the possibility of interval measures of stimuli. Results: The FACES presented sizeable deviations from linearity (equal intervals) in younger children (6-8 years old), which reduced its dynamic range of variation. The scale became more linear in the samples of older children (9-11 years old), especially in the group of children with chronic pain. Conclusions: The FACES scores should not be considered interval measures in children under 8 years old, but may be taken as an approximation to that in children older than 8 years with a history of chronic pain.