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MEASURES OFMEASURES OF

DISEASEDISEASE

FREQUENCYFREQUENCY

A PresentationA Presentation

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EPIDEMIOLOGYEPIDEMIOLOGYis the studyis the studyof frequency, distributions andof frequency, distributions and

determinants of health related statesdeterminants of health related states

and events in human populations, andand events in human populations, and

the application of this study to controlthe application of this study to controlhealth problems.health problems.

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To investigate distributions andTo investigate distributions anddeterminants of disease, it isdeterminants of disease, it is

necessary to know:necessary to know:

The size of the source population from whichThe size of the source population from which

affected individuals were derivedaffected individuals were derived

The time period during which the data wereThe time period during which the data were

collected.collected.

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PROPORTIONPROPORTION

A proportion is a type of ratio in which thoseA proportion is a type of ratio in which those

who are included in the numerator mustwho are included in the numerator must

also be included in the denominator.also be included in the denominator.

For example: the proportion of women overFor example: the proportion of women over

the age of 50 who have had athe age of 50 who have had a

hysterectomy, or the number of fetal deathshysterectomy, or the number of fetal deaths

out of the total number of births (live birthsout of the total number of births (live births

plus fetal deaths).plus fetal deaths).

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RATERATEA rate is a proportion with specificationsA rate is a proportion with specifications

of time. It is a ratio in which there is aof time. It is a ratio in which there is adistinct relationship between thedistinct relationship between the

numerator and denominator with anumerator and denominator with a

measure of time being an intrinsic part ofmeasure of time being an intrinsic part ofthe denominator.the denominator.

For example, the number of newlyFor example, the number of newlydiagnosed cases of breast cancer perdiagnosed cases of breast cancer per

100,000 women during a given year.100,000 women during a given year.

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IMPORTANT POINTIMPORTANT POINT

It is necessary to be very specific about what constitutes both theIt is necessary to be very specific about what constitutes both the

numerator and the denominator. In some circumstances, it isnumerator and the denominator. In some circumstances, it is

important to make clear whether the measure represents theimportant to make clear whether the measure represents the

number of events or the number of individuals.number of events or the number of individuals. For example, the frequency of myopia among a population ofFor example, the frequency of myopia among a population of

school children could represent the number of affected eyes inschool children could represent the number of affected eyes in

relation to total eyes, or the number of children affected in one orrelation to total eyes, or the number of children affected in one or

both eyes relative to all students.both eyes relative to all students.

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PREVALENCEPREVALENCE Prevalence quantifies the proportion of individualsPrevalence quantifies the proportion of individuals

in a population who have the disease at a specificin a population who have the disease at a specific

instant and provides an estimate of the probabilityinstant and provides an estimate of the probability

(risk) that an individual will be ill at a point in time(risk) that an individual will be ill at a point in time

It is a measure of current status of the disease.It is a measure of current status of the disease.

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PREVALENCEPREVALENCE

TheThe formula for calculating the prevalenceformula for calculating the prevalence

P = number of existing cases of a diseaseP = number of existing cases of a disease

divided bydivided by total population (at a giventotal population (at a given

point in time)point in time)

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INCIDENCE:INCIDENCE:

Incidence quantifies the number of newIncidence quantifies the number of new

events or cases of disease that develop inevents or cases of disease that develop in

a population of individuals at risk during aa population of individuals at risk during a

specified time interval.specified time interval.

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INCIDENCE:INCIDENCE:

The number of new events or cases of aThe number of new events or cases of a

specific disease during a given timespecific disease during a given time

divided bydivided by Population at risk [during aPopulation at risk [during a

specified time interval.]specified time interval.]

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Cumulative incidence (CI)Cumulative incidence (CI)

is the proportion of people who becomeis the proportion of people who become

diseased during a specified period of time.diseased during a specified period of time.

It provides an estimate of the probability,It provides an estimate of the probability,

or risk, that an individual will develop aor risk, that an individual will develop a

disease during a specified period of' timedisease during a specified period of' time

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Mid Interval PopulationMid Interval Population

Means the population in the middle of thatMeans the population in the middle of that

specified period e.g. for a whole year, 1stspecified period e.g. for a whole year, 1st

July is mid interval.July is mid interval.

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Issues in the Calculation ofIssues in the Calculation of

Measures of IncidenceMeasures of Incidence For any measure of disease frequency,For any measure of disease frequency,

precise definition of the denominator isprecise definition of the denominator is

essential for both accuracy and clarity.essential for both accuracy and clarity.

This is a particular concern in theThis is a particular concern in the

calculation of incidence. The denominatorcalculation of incidence. The denominator

of a measure of incidence should includeof a measure of incidence should include

only those who are considered "at risk" ofonly those who are considered "at risk" ofdeveloping the disease.developing the disease.

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Contd.Contd.

That is, the total population from which theThat is, the total population from which the

new cases could arise. Consequently,new cases could arise. Consequently,

those who currently have or have alreadythose who currently have or have already

had the disease under study or personshad the disease under study or persons

who cannot develop the disease forwho cannot develop the disease for

reasons such as age, immunization, orreasons such as age, immunization, or

prior removal of the involved organ shouldprior removal of the involved organ shouldbe excluded from the denominator.be excluded from the denominator.

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MORBIDITY RATEMORBIDITY RATE

is the number of nonfatal cases in the total population at riskis the number of nonfatal cases in the total population at risk

during a specified period of time.during a specified period of time.

For example, the morbidity rate of tuberculosis (TB) in the U.S.For example, the morbidity rate of tuberculosis (TB) in the U.S.

in 1982 can be calculated by dividing the number of nonfatalin 1982 can be calculated by dividing the number of nonfatal

cases newly reported during that year by the total U.S. midyearcases newly reported during that year by the total U.S. midyear

populationpopulation

Morbidity rate of TB = 25,5201231,534,000/year = 11.01101/YearMorbidity rate of TB = 25,5201231,534,000/year = 11.01101/Year

Thus, in 1982, the morbidity rate or incidence rate of nonfatalThus, in 1982, the morbidity rate or incidence rate of nonfatal

TB in the United States was 1 1.0 per 100,000 population.TB in the United States was 1 1.0 per 100,000 population.

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MORTALITY RATEMORTALITY RATE

It expresses the number of deaths inIt expresses the number of deaths in

a particular population during a perioda particular population during a period

of time.of time.

It is calculatedIt is calculated

the number of fatalitiesthe number of fatalities divided bydivided by thethe

total population. during that period.total population. during that period.

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Proportionate Mortality RateProportionate Mortality Rate

It measures the proportions of deathsIt measures the proportions of deaths

from a specified cause relative to allfrom a specified cause relative to all

deaths in a particular population duringdeaths in a particular population during

a period of time.a period of time. It is calculatedIt is calculated

the number of fatalities due to athe number of fatalities due to a

specified diseasespecified disease divided bydivided by the totalthe totalnumber of deaths reported during thatnumber of deaths reported during that

period.period.

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