Recent studies have shown that students often misinterpret the area of the box in box plots as representing the frequency or proportion of observations in that interval, while it actually represents density. This misinterpretation has been shown to be based on the saliency of this area and can be explained by heuristic reasoning as defined by dual process theories. In this study we tested whether expert users of box plots also display this misinterpretation and show signs of the same heuristic reasoning as found in students. Using a reaction time test, we found signs of heuristic reasoning in experts, both with respect to accuracy and reaction times. If even experts have difficulty interpreting box plots, one can question whether these are an appropriate form of representation to use when reporting data and deserve the prominent place they currently have in the statistics curriculum.
Keywords: Box plots; Dual process theories; Experts; Heuristic reasoning.