Qualitative descriptors of disease incidence: commonly used and frequently muddled.

Words such as "frequent," "common," "uncommon," and "rare" are used qualitatively to discuss disease incidence. They give a subjective assessment of disease frequency and are, thus, open to interpretation. A review of major journals in pediatrics and medical genetics was undertaken to determine whether these terms are applied with sufficient consistency within and/or between medical specialties to confer useful information. An article was included in the study if it used a keyword as a general descriptor (X is common), but not if it was used as a comparator (Y is the most common). Bibliographical information, the disease, the keyword, and the incidence of the disease were recorded for each article that met the search criteria. If the disease population incidence or prevalence was not mentioned in the article, another literature or database source was used to obtain that number. The results showed extreme inconsistency in the use of the terms and that the incidence ranges overlap significantly. There is disagreement about the meaning of the words both between specialties and within the same specialty. Editorial boards might consider setting parameters for use of these terms in order to decrease confusion about how "common" a condition actually is.