Abstract
As the methods for the functional analysis of problem behavior have continued to develop, there has been a greater focus on the specificity of controlling variables, both antecedents and consequences. Accelerating research interest in the role of antecedents reveals that a large array of stimulus variables can influence the rate of problem behavior. Indeed, the variety of these stimuli is so great that it is sometimes possible to overlook specific stimulus variables during initial assessment. The present study shows that a failure to identify these very specific (idiosyncratic) stimulus variables is serious because their presence can systematically alter the outcomes of functional analyses that are designed to assess the motivation of problem behavior. Guidelines are therefore discussed concerning when to suspect that idiosyncratic stimuli might be acting to influence assessment data, thereby promoting a search for additional stimulus variables whose identification can aid in improving the design of functional analysis conditions.