The Limits of Granularity Adjustments
by Jean-David Fermanian of CREST-ENSAE
March 20, 2013
Abstract: We provide a rigorous proof of granularity adjustment (GA) formulas to evaluate loss distributions and risk measures (value-at-risk) in the case of heterogenous portfolios, multiple systemic factors and random recoveries. As a significant improvement with respect to the literature, we detail all the technical conditions of validity and provide an upper bound of the remainder term at a finite distance. Moreover, we deal explicitly with the case of general loss distributions, possibly with masses. For some simple portfolio models, we prove empirically that the granularity adjustments do not always improve the infinitely granular first-order approximations. This stresses the importance of checking our conditions of regularity before relying of such techniques. And smoothing the underlying loss distributions through random recoveries or exposures improves the GA performances in general.
Keywords: Credit portfolio model, Granularity adjustment, Value-at-risk, Fourier Transform.