CONCEPTUAL LIMITATIONS IN COMPREHENDING LINE GRAPHS

This article examines the conceptual limitations that influence a viewer's mental representation of a data set of 3 continuous variables. Such data can be graphed in 2 ways, depending on which variable is depicted on the x axis and which is made a parameter on the curve (the z variable). The results of several studies indicate that a viewer's internal representation of a graph expresses quantitative functional information about the x-y relations (such as ''y decreases exponentially as x increases''). By contrast, the z-y information is represented ordinally or even nominally. This difference in the mental representations of the variables was evident when viewers described or interpreted simple line graphs or judged the equivalence of successively presented graphs. The difference persisted in spite of the viewer's accurate encoding of the graph's visual pattern, as indicated by accuracy in reproducing them from memory. Viewers seldom formed an integrated representation of the 3 variables, even when explicitly instructed to encode them. The x-y line patterns may automatically activate quantitative concepts, whereas viewers may be unfamiliar with the quantitative interpretation of z-y patterns.