统计问题（30）：诊断出多发性硬化症后的平均生存期比诊断出宫颈癌后长得多。它们的发病率几乎相同，每年每 100000 人中有 5 人，尽管这因地理和人口统计而异。在大量人群中，多发性硬化症的患病率预计为： QuestionThe average survival after a diagnosis of multiple sclerosis is much longer than after a diagnosis of cervical cancer. They both have about the same incidence of five per 100 000 per year, although this varies with geography and demography. In a large population the prevalence of multiple sclerosis would then be expected to be:a) The same as that of cervical cancerb) Much less than that of cervical cancerc) Much greater than that of cervical cancerd) Less than the incidence of multiple sclerosise) Greater than the incidence of multiple sclerosisAnswerIncidence is measured in units of number of cases in a population in a time period. Prevalence has no units and is simply a proportion—the number of existing cases in a population at a given time. Because prevalence and incidence are different and measured with different units it makes little sense to compare their numerical values as in d and e.In a stable population with stable disease characteristics prevalence is equal to disease incidence multiplied by average duration of disease. Because people live much longer on average after a diagnosis of multiple sclerosis than after a diagnosis of cervical cancer, the prevalence of multiple sclerosis is much greater than that of cervical cancer.中文解释：发病率以一段时间内人口中的病例数为单位来衡量。患病率没有单位，只是一个比例——特定时间人口中现有病例的数量。因为患病率和发病率概念不同，并且用不同的单位进行测量，所以单纯比较它们的数值几乎没有意义（d,e）。在具有稳定疾病特征的稳定人群中，患病率等于疾病发病率乘以平均病程。由于诊断出多发性硬化症后的平均寿命比诊断出宫颈癌后的平均寿命要长得多，因此多发性硬化症的患病率远高于宫颈癌。所以答案是选择 C
