They are ranked according to system organ class and classified according to the following convention: very common (≥ 1/ 10), common (≥ 1/ 100 to < 1/ 10), uncommon (≥ 1/ 1000 to < 1/ 100), rare (≥ 1/ 10,000 to < 1/ 1000), very rare (< 1/ 10,000), or not known (cannot be estimated from the available data).