On time-scaling of risk and the square–root–of–time rule

Jean-Pierre Zigrand () and Jon Danielsson

FMG Discussion Papers from Financial Markets Group

Abstract: Many financial applications, such as risk analysis and derivatives pricing, depend on time scaling of risk. A common method for this purpose, though only correct when returns are iid normal, is the square root of time rule where an estimated quantile of a return distribution is scaled to a lower frequency by the square-root of the time horizon. The aim of this paper is to examine time scaling of risk when returns follow a jump diffusion process. It is argued that a jump diffusion is well-suited for the modeling of systemic risk, which is the raison d'etre of the Basel capital adequacy proposals. We demonstrate that the square root of time rule leads to a systematic underestimation of risk, whereby the degree of underestimation worsens with the time horizon,the jump intensity and the confidence level. As a result,even if the square root of time rule has widespread applications in the Basel Accords, it fails to address the objective of the Accords.

Date: 2003-03
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Related works:
Journal Article: On time-scaling of risk and the square-root-of-time rule (2006)
Working Paper: On time-scaling of risk and the square–root–of–time rule (2003)
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