(15)Default rates should be calculated for each rating category to the extent possible over a long term and a short term observation period. The former should provide the basis for the mapping, whereas the latter should provide an early warning about a potential increase, or decrease, in the level of risk of the rating category. Where a sufficient number of credit ratings is not available, only the long run default rate should be calculated due to the high degree of uncertainty regarding the calculation of short run default rates. In this case, a warning about a potential increase in the level of risk of the rating category should be provided by the qualitative factors.