For purposes of determining the base interest rate, which is often described in practice as the risk-free rate or base rate, the yield curve as referred to in IDW S1 (IDW: Institute of Public Auditors in Germany) is often relied upon for practical purposes and is to be seen as the best estimator of market performance according to the IDW. This is determined on the basis of parameters published each trading day by the Deutsche Bundesbank since 1997 based on the Svensson Method. For purposes correcting for potential market fluctuations and to compensate for any estimation errors, we have computed a three-month average interest rate as of the relevant issue date when calculating sector betas in conformance with IDW. This was done using a uniform base interest rate based on an equivalent present value.
WOLLNY WP has created the base interest rate calculator BaseRateGuide® with enables the yield curve and the base interest rate to be calculated precisely as of a given date. You may find the BaseRateGuide® here.