To measure systemic stability, a number of studies attempt to aggregate firm-level stability measures (z-score and distance to default) into a system-wide evaluation of stability, either by taking a simple average or weighing each measure by the institution's relative size. However, these aggregate measures fail to account for correlated risks among financial institutions. In other words, the model fails to consider the inter-connectedness between institutions, and that one institution's failure can lead to a contagion.