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**Measures of Disease Frequency**

Nigel Paneth

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**FRACTIONS USED IN DESCRIBING DISEASE FREQUENCY**

RATIO A fraction in which the numerator is not part of the denominator. e.g. Fetal death ratio: Fetal deaths/live births. Fetal deaths are not included among live births, by definition.

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PROPORTION A fraction in which the numerator is part of the denominator. e.g. Fetal death rate: Fetal deaths/all births All births includes both live births and fetal deaths. Synonyms for proportions are: a risk and, (if expressed per 100) a percentage. Most fractions in epidemiology are proportions.

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RATE Ideally, a proportion in which change over time is considered, but in practice, often used interchangeably with proportion, without reference to time, (as I did previously for fetal death rate).

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**MEASURES OF DISEASE FREQUENCY**

PREVALENCE RATE Divided into two types: Point prevalence rate Period prevalence rate

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POINT PREVALENCE RATE Proportion of individuals in a specified population at risk who have the disease of interest at a given point in time.

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**PERIOD PREVALENCE RATE**

Proportion of individuals in a specified population at risk who have the disease of interest over a specified period of time. For example: annual prevalence rate lifetime prevalence rate. (When the type of prevalence rate is not specified it is usually point prevalence, or its closest practical approximation)

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**1. Cumulative incidence rate 2. Incidence density**

Like prevalence, divided into two types: 1. Cumulative incidence rate 2. Incidence density

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**1. Cumulative incidence rate:**

Number of new cases of disease occurring over a specified period of time in a population at risk at the beginning of the interval.

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**EXAMPLE OF CUMULATIVE INCIDENCE RATE**

If we count all new cases of influenza occurring in MSU undergraduates from September 1, August 31, 1998, and we take as the denominator all undergraduates enrolled in September 1, 1997, we would be describing the cumulative incidence rate of influenza.

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2. Incidence density: Number of new cases of disease occurring over a specified period of time in a population at risk throughout the interval.

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**The numerator does not differ between the two types of incidence **

However, the denominator can differ in incidence density from cumulative incidence because it takes account of (in the example): Students who left school during the year Students who died Students who had influenza once and will not have it again the same season Students who entered school later in the year

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Incidence density requires us to add up the period of time each individual was present in the population, and was at risk of becoming a new case of disease. Incidence density characteristically uses as the denominator person-years at risk. (Time period can be person-months, days, or even hours, depending on the disease process being studied.)

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The mortality rate is a type of incidence rate which we will discuss in more detail next week. It is the most widely used public health measure, but it has limitations.

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**USES OF INCIDENCE AND PREVALENCE**

Incidence is generally used for acutely acquired diseases, prevalence is used for more permanent states, conditions or attributes of ill-health. Incidence is more important when thinking of etiology of the disorder, prevalence when thinking of societal burden of the disorder including the costs and resources consumed as a result of the disorder. Incidence always requires a duration, prevalence may or may not.

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In incidence, the unit of analysis is the event, in prevalence, it is the person. Thus incidence may exceed 100% (e.g. annual incidence of colds) unless a convention is adopted to count only first episodes of an illness that can occur more than once. Prevalence can never exceed 100%. Incidence generally requires an initial disease-free interval before counting starts, because incidence is measured only in those at-risk of disease.

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**USES OF INCIDENCE DENSITY AND CUMULATIVE INCIDENCE**

Incidence density gives the best estimate of the true risk of acquiring disease at any moment in time. Cumulative incidence gives the best estimate of how many people will eventually get the disease in an enumerated population.

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**RELATIONSHIP BETWEEN INCIDENCE AND PREVALENCE**

In a STEADY STATE (i.e. if incidence is not changing, and the population is stable) Prevalence rate = incidence rate times the duration of disease (P = I x D)

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