An Investigation into the Measurement of Graph Distortion in Financial Reports

The Graph Discrepancy Index (GDI), which originates from the lie factor introduced by Tufte (1983), is the mechanism commonly used in the financial graphics literature to determine whether graphs are distorted and to quantify the extent of such distortion. Whilst the GDI is critical to the financial graphics literature, little or no attention has been paid to its robustness and accuracy. We critically examine the mathematical characteristics of the GDI and show its limitations as a measure of graph distortion. We review a number of cases to demonstrate these limitations. We present an alternative measure of graph distortion, the Relative Graph Discrepancy index (RGD). Numerous simulations suggest that the RGD overcomes the problems associated with the GDI. The RGD is also tested on data presented in earlier research and the results are compared to those obtained using the GDI. In comparison with the GDI we find that the RGD is more consistent and produces slightly stronger results. We stress however that this is not a best or definitive measure but is intended to start a research process that leads to a generally accepted measure