The thresholds of change model: An approach to analyzing stages of change data

Stage models are prominent in research describing health behavior change. S ince stage models often propose that different factors have varying influen ces on membership in the different stages, statistical methods that can est imate the thresholds that separate the stages and the relative value of var iables in influencing these thresholds are useful. This article describes u se of a "thresholds of change" model for analyzing the thresholds separatin g stages and specifically for examining the effects of explanatory variable s on these thresholds using a generalization of an ordinal logistic (or pro bit) regression model. Data from a skin cancer prevention study (N = 3,185) in which participants were grouped into three stages for sunscreen use (pr econtemplation, contemplation, and action) are used to illustrate the Thres holds of Change Model. For this example, two thresholds exist: a contemplat ion (between precontemplation and contemplation) and an action threshold (b etween contemplation and action). Variables examined include gender skin ty pe, perceived susceptibility to sunburn, worry about skin cancel; and sun p rotection self-efficacy We examine models that assume that the effects of t hese variables are the same across thresholds, and then allow the effects o f these variables to van, across thresholds. Results indicate that perceive d susceptibility has an equal effect on both thresholds, but that worry and self-efficacy have differential effects: worry exerts a greater influence on the contemplation threshold whereas self-efficacy has a significantly st ronger effect on the action threshold Gender also has a stranger effect on the action threshold; males were less likely to be classified in the action stage than females. This analytic approach has broad applications to many types of stage delta.