JUDGING PROPORTION WITH GRAPHS - THE SUMMATION MODEL

People take longer to judge part-to-whole relationships with bar graph s than with pie charts or divided bar graphs. Subjects may perform sum mation operations to establish the whole with bar graphs, which would be unnecessary for other graph types depicting the whole with a single object. To test this summation model, the number of components formin g the whole was varied with bars, divided bars, reference bars, and pi es in three experiments. Response time increased with the number of co mponents for bar graphs but there was little increase for other graph types in Experiment 1. An accuracy emphasis in Experiment 2 produced g enerally longer response times, but had little effect on the time per summation. The summation operation was not used when graphs were displ ayed briefly in Experiment 3, although subjects still look longer with bars. The estimated time for a summation operation is consistent with estimates derived from other research. In general, the bar graph is n ot effective for proportion judgments, and its disadvantage becomes po tentially greater as the number of components increases. (C) 1998 John Wiley & Sons, Ltd.