(8)To take into account the effect of possible large changes to trading book positions, which typically happen on a daily basis, institutions should calculate the required risk numbers on the basis of an observation period of 15 consecutive business days, rather than on the basis of a single point in time. However, to include a certain degree of proportionality in the assessment of whether changes to the use of alternative internal models and changes to the subset of the modellable risk factors are material, that observation period of 15 consecutive business days should be subject to exemptions where the assessed quantitative impact is very minor on the first testing date and where there is a presumption that the quantitative thresholds will not be breached during that 15 consecutive business days period.