Ví dụ: Tính toán Tỷ lệ Trong một cuộc khảo sát 1,150 phụ nữ ở Maine Gave sinh năm 2000, tổng cộng 468 báo cáo tham Multivitamin ít nhất 4 lần một tuần trong tháng trước khi trở thành pregnant. | EXAMPLE Calculating Prevalence In a survey of 1 150 women who gave birth in Maine in 2000 a total of468 reported taking a multivitamin at least 4 times a week during the month before becoming Calculate the prevalence of frequent multivitamin use in this group. Numerator 468 multivitamin users Denominator 1 150 women _Prevalence 468 1 150 x 100 x 100 _ Properties and uses of prevalence Prevalence and incidence are frequently confused. Prevalence refers to proportion of persons who have a condition at or during a particular time period whereas incidence refers to the proportion or rate of persons who develop a condition during a particular time period. So prevalence and incidence are similar but prevalence includes new and pre-existing cases whereas incidence includes new cases only. The key difference is in their numerators. Numerator of incidence new cases that occurred during a given time period Numerator of prevalence all cases present during a given time period The numerator of an incidence proportion or rate consists only of persons whose illness began during the specified interval. The numerator for prevalence includes all persons ill from a specified cause during the specified interval regardless of when the illness began. It includes not only new cases but also preexisting cases representing persons who remained ill during some portion of the specified interval. Prevalence is based on both incidence and duration of illness. High prevalence of a disease within a population might reflect high incidence or prolonged survival without cure or both. Conversely low prevalence might indicate low incidence a rapidly fatal process or rapid recovery. Prevalence rather than incidence is often measured for chronic diseases such as diabetes or osteoarthritis which have long duration and dates of onset that are difficult to pinpoint. This is trial version Measures of Risk Page 3-17 .
