The Use of Incidence and Prevalence in the Study of Disease Development in a Population

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Abstract
A mathematical model is presented of the interrelationships of prevalence and incidence of a disease in a population for diseases that have a chronic and protracted course. Incidence is measured at a point in disease-development time and is a count of cases of disease reaching this point over a unit of calendar time. Prevalence is measured at a point of calendar time and is a count of cases representing an interval of disease-development time. The interval of disease-development time represented by prevalence is bounded by 2 points from which incidence can be determined. The prevalence of disease representing an interval of disease - development time and the incidence of disease at either of the 2 points bounding this interval are quantitively related. Knowing any 2 the 3rd can be determined. The ratio of prevalence to incidence rates taken over all ages of the population by the use of unweighted age-specific rates gives a measure of the average length of the prevalence interval in the calendar time units represented by incidence. Prevalent disease can be looked upon as a pool of disease which is static in size but is constantly being fed by new disease entering the pool and constantly depleted by maturing disease leaving the pool. Viewing the distribution of the prevalence pool over the population by age with age-specific rates, and the age-specific rates of additions or depletions to the prevalence pool indicates whether there is variation in average length of the prevalence interval at different ages in the population. Use of the method is described in considering the relationship of carcinoma in situ to invasive cancer of the cervix. The method as a means of measuring the efficiency of a case-finding procedure is illustrated with data from X-ray screening of a male population for lung cancer.
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