Graphs versus tables: Effects of data presentation format on judgemental forecasting

We report two experiments designed to study the effect of data presentation format on the accuracy of judgemental forecasts. In the first one, people studied 44 different 20-point time series and forecast the 21st and 22nd points of each one. Half the series were presented graphically and half were in tabular form. Root mean square error (RMSE) in forecasts was decomposed into constant error (to measure bias) and variable error (to measure inconsistency). For untrended data, RMSE was somewhat higher with graphical presentation: inconsistency and an overforecasting bias were both greater with this format. For trended data, RMSE was higher with tabular presentation. This was because underestimation of trends with this format was so much greater than with graphical presentation that it overwhelmed the smaller but opposing effects that were observed with untrended series. In the second experiment, series were more variable but very similar results were obtained.