Period prevalence refers to prevalence measured over an interval of time It is

# Period prevalence refers to prevalence measured over

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Period prevalence refers to prevalence measured over an interval of time. It is the proportion of persons with a particular disease or attribute at any time during the interval. Method for calculating prevalence of disease Method for calculating prevalence of an attribute The value of 10 is usually 1 or 100 for common attributes. The value of 10 might be 1,000, 100,000, or even 1,000,000 for rare attributes and for most diseases. EXAMPLE: Calculating Prevalence In a survey of 1,150 women who gave birth in Maine in 2000, a total of 468 reported taking a multivitamin at least 4 times a week during the month before becoming pregnant.( 7 ) Calculate the prevalence of frequent multivitamin use in this group. Numerator = 468 multivitamin users Denominator = 1,150 women Prevalence = (468 ⁄ 1,150) × 100 = 0.407 × 100 = 40.7% 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 di²erence 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 speci±ed interval. The numerator for prevalence includes all persons ill from a speci±ed cause during the speci±ed 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 speci±ed interval. Prevalence is based on both incidence and duration of illness. High prevalence of a disease within a population might re³ect 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 di´cult to pinpoint. EXAMPLES: Incidence versus Prevalence Figure 3.1 represents 10 new cases of illness over about 15 months in a population of 20 persons. Each horizontal line represents one person. The down arrow indicates the date of onset of illness. The solid line represents the duration of illness. The up arrow and the cross represent the date of recovery and date of death, respectively. Figure 3.1 New Cases of Illness from October 1, 2004– September 30, 2005 Image Description Example A: Calculate the incidence rate from October 1, 2004, to September 30, 2005, using the midpoint population (population alive on April 1, 2005) as the denominator. Express the rate per 100 population.

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