(2)The expected shortfall risk measure should capture the probability distribution of risk factors over a sufficiently long historical period in which the relevant market data for those risk factors are observable. Therefore, a risk factor should be considered modellable where a sufficient number of observable verifiable prices that are representative of that risk factor are available. To perform that assessment, a 12-month observation period ending at the previous reporting reference date should be appropriate. However, to take into account possible delays in data availability, institutions should be allowed to replace that 12-month observation period with a shifted 12-month period. To ensure comparability of practices across the Union, such shift should be limited to one month. For the same reason, institutions should apply such shifted periods consistently across all risk factors of the same type and provide their competent authority with detailed documentation as regards the application of such shifted periods.