Only duties and imports recorded under HS Chapters 01-97 are taken into account. Each applied tariff schedule is validated against the standard nomenclature at the HS six-digit subheading level of the HS version adopted by the country for the reference year. National tariff lines that do not follow this standard (i.e., the first six digits should be based on the standard subheading nomenclature of the HS version used by the country) are discarded and not taken into account. On the other hand, missing subheadings are added. Hence, all calculations are based on the complete standard nomenclature. All simple averages are based on pre-aggregated HS six-digit averages. Pre-aggregation means that duties at the tariff line level are first averaged to HS six-digit subheadings. Subsequent calculations are based on these pre-aggregated averages. To the extent possible, non-ad valorem duties are converted into ad valorem equivalents. The methodology used for the conversion is in Technical Annex B of World Tariff Profiles 2006.