Their frequency is defined using the following conventions: common (≥1/100 to < 1/10); uncommon (≥1/1,000 to < 1/100); rare (≥1/10,000 to < 1/1,000); very rare (< 1/10,000).
Nende esinemissagedus on defineeritud järgmiselt: sage (> 1/ 100, < 1/ 10); aeg- ajalt (> 1/ 1000, < 1/ 100); harv (> 1/ 10000, < 1/ 1000); väga harv (< 1/ 10000).