(24)Rules should be established to specify the appropriate percentage and time horizons for the liquidation period and the calculation of historical volatility. However, in order to ensure that CCPs duly manage the risk they face, it is desirable not to specify the approach which the CCP should take to calculating margin requirements from these parameters. For the same reasons, CCPs should not be prevented to rely on various reliable methodological approaches for the development of portfolio margining, they should be allowed to rely on methods based on correlations between price risk of the financial instrument or set of financial instruments they clear, as well as any appropriate methods based on equivalent statistical parameters of dependence.