Measures of Disease Frequency 0903_gaohongcai(2)

8/14/2019 Measures of Disease Frequency 0903_gaohongcai(2)

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Me asu res o fDise ase sFrequency(2)Gao Hongcai

Email: [email protected]

School of Public Health, Jining MedicalCollege


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Mortality Frequency

Measures


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Mortality Measures Crude Mortality Rate : is a measure of the

frequency of occurrence of death in a definedpopulation during a specified interval.

i.e. for a defined population over a specified periodof time

k=occurreddeathstheamongpopulationmidTotal

periodgiven timeaduringoccurringDeathMRCrude

k=1,000 or

100,000

The crude mortality rate is the mortality rate fromall causes of death for a population.


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Mortality Measures

yearperpopulation1000perdeaths61000

2000000

12000MRCrude ==

Example:In 2001, there were 12,000 deaths from

all causes among 2 million population of Atlanta,Georgia.


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Mortality Measures Case Fatality Rate:is the proportion of

persons with a particular condition (cases) that

die from the condition. It is usually expressed inpercent.k=

diseasesamewith thepeopleofNumber

diseaseatoduedeathsofNumbertyRateCaseFatali

k=100

It is simply the ratio ofcause-specific deaths tocases during a specified time,

It requires that the deaths in the numerator belimited to the cases in the denominator.


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Mortality Measures Example: in Richmond city, there were 500

cases of colon cancer, of whom five died

within a year after their initial diagnosis. Thecase fatality rate for colon cancer inRichmond city was:

5/500 x 100 = 1%


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Mortality Measures Age-specific MR= is a mortality

rate limited to a particular age group. The numerator is the number of deaths

in that age group;

the denominator is the number of

persons in that age group in thepopulation.

Some specific types of age-specificmortality rates are neonatal,postneonatal,

andinfant mortality rates.


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Mortality Measures Infant Mortality Rate :is one of the most

commonly used measures for comparing

health service among nations.

k=yearsametheduringbirthsliveofnumberTotal

ageofyearonethanlessdeathsinfantofNumber

IMR

k=1000


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Mortality Measures Infant Mortality Rate The numeratoris the number of deaths

among children under 1 year of agereported during a given time period,usually a calendar year.

The denominatoris the number of livebirths reported during the same time

period.

The infant mortality rate is usually

expressed per 1,000 live births.


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Mortality MeasuresIs the infant mortality rate aproportion?

Technically, it is a ratio but nota proportion.

Why?


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Mortality MeasuresAn example:

Consider the U.S infant mortality rate for

1988. In 1988, 38,910 infants died and 3.9million children were born,

for an infant mortality rate of 9.95 per1,000 .

Undoubtedly, some of these deathsoccurred among children born in 1987, butthe denominator includes only children

born in 1988.


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Mortality Measures Example: Denver, Colorado, with a

population of 2.37 million, reported a

total of 270 infant deaths and 30,000live births in 1981.

IMR = (270/30,000) x 1000 = 9 infant

deaths per 1000 live births per year.


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Mortality Measures Neonatal Mortality Rate :The neonatal

periodis defined as the period from birth up

to 28 days.k=

yearsametheduringbirthsliveofnumberTotal

periodgiven timeaduringageofdays28underchildrenamongdeathsofnumberNMR

k=1000


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Mortality Measures E.g. Memphis, Tennessee, with a population

of 1.37 million, reported a total number of150 neonatal deaths and 30,000 live births in1982.

calculate NMR.NMR = (150/30,000 )x 1000 =

5%

Five neonatal deaths per 1000 live birthsper year.


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Mortality Measures Post neonatal mortality rate :The post

neonatal period is defined as the period from 28

days of age up to but not including 1 year of age.

kPNMR =periodtimesametheduringbirthsliveofNumber

ageofyr.1anddays28betweenchildrenamongdeathsofNo.

k=1000

Notice: is the number of deaths amongchildren from 28 days up to but not

including 1 year of age during a giventime period.


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Mortality Measures

Example: In Oakland California there were 2

million births in 1995, of which 10,000 died

before they reach their first birth day. 7,500of the infant died within the first 28 days.

Calculate the IMR, NMR and PNMR

The postneonatal mortality rate is usuallyexpressed per 1,000 live births. Live

birthsInfant deaths

Neonataldeaths


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Maternal mortality rate

The maternal mortality rate is really aratio used to measure mortalityassociated with pregnancy.

The numerator is the number ofdeaths assigned to causes related topregnancy during a given time period.


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Maternal mortality rate

The denominator is the number of live

births reported during the same timeperiod.

Because maternal mortality is much

less common than infant mortality, thematernal mortality rate is usuallyexpressed per 100,000 live births.

e.g. In 1988, the maternal mortality rate

was 8.4 per 100,000 live births (7).


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Cause-specific mortality rate

The cause-specific mortality rate is the

mortality rate from a specified cause for apopulation.

The numerator is the number of deaths

attributed to a specific cause. The denominator remains the size of the

population at the midpoint of the timeperiod.

n


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Sex-specific mortality rate

A sex-specific mortality rate is a

mortality rate among either malesor females.

Both numerator and denominator

are limited to the one sex.


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Race-specific mortality rate

A race-specific mortality rate is a

mortality rate limited to a specifiedracial group.

Both numerator and denominator

are limited to the specified race.


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Combinations of specific mortalityrates

Mortality rates can be further refinedto combinations that are cause-specific, age-specific, sex-specific,and/or race-specific.


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Combinations of specific mortality rates

For example,

the mortality rate attributed to HIV among25- to 44-year-olds in the United States in1987 was 9,820 deaths among 77.6 million25- to 44-year olds, or 12.7 per 100,000.

This is a cause- and age-specific mortalityrate, because it is limited to one cause (HIVinfection) and one age group (25 to 44years).


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Age-adjusted mortality rates

Often, we want to compare themortality experience of differentpopulations. However, since mortality

rates increase with age, a highermortality rate in one population than inanother may simply reflect that the

first population is older than the


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Age-adjusted mortality rates

Statistical techniques are used to

adjustor standardizethe rates in thepopulations to be compared whicheliminates the effect of different agedistributions in the differentpopulations.

Mortality rates computed with thesetechniques are called age-adjustedora e-standardized mortalit rates.


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Example

A total of2,123,323 deaths were

recorded in the United States in 1987. The mid-year population was estimated

to be 243,401,000.

HIV-related mortality and populationdata by age for all residents and forblack males are shown inTable3.


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Table 3.


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We will use these data to calculate thefollowing four mortality rates:

a. Crude mortality rate b. HIV-(cause)-specific mortality rate for

the entire population

c. HIV-specific mortality among 35- to44-year-olds

d. HIV-specific mortality among 35- to44-year-old black males


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a. Crude mortality rate

= (Number of deaths in the U.S./Totalpopulation) 100,000

= (2 123 323/243 401 000) 100,000

= 872.4 deaths per 100,000 population


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b. HIV (cause)-specific mortality rate for th

=

Population

deathsHIVofNumber

=(13 468/243 401 000) 100,000

= 5.5 HIV-related deaths per 100,000

population

10n


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c. HIV-related mortality rate among 35

Number of HIV deaths in 35 - to 44 - year -olds Population of 35 - to 44 - year olds

10n=

4 79434 305 000

10n=

= 14.0 HIV-related deaths per 100,000 35- to 44-year-olds


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d.HIV-related mortality rate among 35- to 44-year-old blac

(cause-, age-, race-, and sex-specific mortality rate)

10nNumber of HIV deaths in 35 - to 44 - year - old blackmalesPopulation of 35 - to 44 - year - old black males

=

1 2121 663 000

10n=

72.9 HIV-related deaths per 100,000 35- to 44-year-oldblack males


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Death-to-case ratio

The death-to-case ratio is the number

of deaths attributed to a particulardisease during a specified time perioddivided by the number of new cases ofthat disease identified during the same

time period:k=

periodsameduringidentifieddiseasetheofcasesnewofNumber

periodspecifiedduringdiseasesparticularofdeathsofNumberratiocase-to-Death


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Death-to-case ratio

The figures used for the numerator and

denominator must apply to the samepopulation.

However, The deaths in the numerator are

NOT NECESSARILY included in thedenominator, because some of the deathsmay have occurred in persons who developedthe disease before the specified period.


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For example, 22,517new cases of

tuberculosis were reported in the

United States in 1987(2). During the

same year, 1,755deaths occurred

that were attributed to tuberculosis.


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Presumably, many of the deaths occurred inpersons who had initially contracted

tuberculosis years earlier. Thus, many of the1,755 in the numerator are not among the22,517 in the denominator.

Therefore, the death-to-case ratio is a ratiobut not a proportion. The tuberculosis

death-to-case ratio for 1987 is:


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We can calculate the number of deathsper 100 cases by dividing the

numerator by the denominator (10n =100 for this calculation):

1.755 22,517 100 = 7.8 deaths per 100 newcases


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Alternatively, we can calculate the numberof cases per death by dividing the

denominator by the numerator (10n

= 1 forthis alculation):

22,517 1,755 = 12.8

Therefore, there was 1 death per 12.8 newcases.

It is correct to use either expression of the

ratio.


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Proportionate mortality

Proportionate mortality describes theproportion of deaths in a specified

population over a period of time attributableto different causes.

Each cause is expressed as a percentage of

all deaths, and the sum of the causes mustadd to 100%.

These proportions are not mortality rates,since the denominator is all deaths, not theo ulation in which the deaths occurred.


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For a specified population over a specifiedperiod,

100causesallfromDeaths

causeparticularatodueDeathsmortalityateProportion =

a e


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a e4.


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Sometimes we compare theproportionate mortality in one age

group or occupational group to theentire population, either for deathsfrom all causes or from a specific

cause.

The resulting ratio is called a

proportionate mortality ratio, or PMR


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Mortality Measures

Years of Potential Life Lost (YPLL)is a measure of the impact of

premature mortality on a population.

Premature death = when a death occurs

earlier than expected if disease were notpresent.


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It is calculated as the sum of the differencesbetween some predetermined end point and

the ages of death for those who died beforethat end point.

The two most commonly used end pointsare age 65 years and average life

expectancy. Because of the way in which YPLL is

calculated, this measure gives more weight

to a death the earlier it occurs.


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Calculating YPLL from a line listing

1. Eliminate the records of all persons

who died at or after the end point (e.g.,age 65 years).

2. For each person who died before the

end point, identify that individuals YPLLby subtracting the age at death from theend point.

3. Sum the YPLLs.


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TheYears of Potential Life Lost Rate

represents years of potential life lost per

1,000 population below the age of 65years (or below the average life

expectancy).

YPLL rates should be used to comparepremature mortality in different

populations, since YPLL does not take into

account differences in population sizes.


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The formula for a YPLL rate is as follows:

k=years65ageunderPopulation

lostlifepotentialofYearsrateYPLL


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Two end points are in common use,

The first, age 65. The 65-year end point

assumes that everyone should live at least

to age 65, and any death before that age is

premature.

It ignores deaths after age 65.

Thus, the 65-year end point emphasizes

causes of death among younger persons.


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The second end point commonly used islife expectancy remaining at the time of

death. Years of potential life lost for each death

is calculated by subtracting the age atdeath (or age group midpoint) from the

remaining life expectancy at that age. The remaining life expectancy is available

from an abridged life table publishedannually by the National Center for Health


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For example, in 1984, the remaining lifeexpectancy for a 60-year-old was 20.4

years, and the remaining life expectancyfor the age group 75 to 84 years was 8.2years. Since deaths at older ages are far

more numerous, the life-expectancymethod for calculating years of potentiallife lost places less emphasis on deathsat early ages, and more closely


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Mortality Measures

survival rate

Indicate the percentage of people in a study or

treatment group who are alive for a certain periodof time after they were diagnosed with or treated

for a disease (such as cancer).

The survival rate is often stated as a five-year

survival rate, which is the percentage of people ina study or treatment group who are alive five

years after diagnosis or treatment.


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Mortality Measures

Survival rates are important forprognosis; for example, whether a

type ofcancer has a good or badprognosis can be determined from itssurvival rate.
http://en.wikipedia.org/wiki/Prognosishttp://en.wikipedia.org/wiki/Cancerhttp://en.wikipedia.org/wiki/Cancerhttp://en.wikipedia.org/wiki/Prognosis


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Mortality Measures

Patients with a certain disease can die

directly from that disease or from an

unrelated cause such as a car accident.

When the precise cause of death is not

specified, this is called the overall

survival rate or observed survival

rate.


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Mortality Measures

Doctors often use mean overall survival

rates to estimate the patient's prognosis.

This is often expressed over standard time

periods, like one, five, and ten years.

For example, prostate cancer has a much

higher one year overall survival rate than

pancreatic cancer, and thus has a better

prognosis.
http://en.wikipedia.org/wiki/Pancreatic_cancerhttp://en.wikipedia.org/wiki/Prostate_cancer


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Mortality Measures

When someone is more interested inhow survival is affected by the disease,

there is also the net survival rate,which filters out the effect of mortalityfrom other causes than the disease.

The two main ways to calculate net

survival relative survival cause specific survival or disease

specific survival.
http://en.wikipedia.org/wiki/Relative_survivalhttp://en.wikipedia.org/wiki/Relative_survival


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Mortality Measures

Relative survival is calculated bydividing the overall survival after

diagnosis of a disease by the survivalas observed in a similar populationthat was not diagnosed with that

disease. A similar population is composed of

individuals with at least age and

gender similar to those diagnosed with


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Mortality Measures

Cause specific survival is calculated bytreating deaths from other causes than

the disease as withdrawals from thepopulation that don't lower survival,comparable to patients who are not

observed any longer, e.g. due toreaching the end of the study period.


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Mortality Measures

Relative survival has theadvantage that it does not depend

on accuracy of the reported causeof death;

cause specific survival has the

advantage that it does not dependon the ability to find a similarpopulation of people without the

disease.


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Natality FrequencyMeasures


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In epidemiology, natality measuresare used in the area of maternal and

child health and less so in other areas.Table 5 shows a summary for some

frequently used measures of natality.


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Table 5.


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Adjusted Rates


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Adjusted Rates

A statistically manipulated rate

controlling for the factor of interest

(age, race, etc)

Specific rates give more valuable

information when comparing ratesbetween sub-groups.

However, specific rates do not provide

accurate comparison with other population.


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Adjusted Rates

Adjusted rates provide a summarystatistic that corrects for differences in age

or other factors in populations makingcomparisons possible between differentpopulations

The computational process for adjustedrate is called standardization oradjustment .


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Adjusted Rates

Advantages Provides summary statement

Differences in groups removed to permitunbiased comparisons

Disadvantages

Statistically manipulated rates (fictional) Absolute magnitude dependent on

choice of reference population


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Adjusted Rates

Two methods of adjustment: 1) Direct

2) Indirect Direct Method is commonly utilized

in practice

But is this necessarily the mostaccurate?


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Adjusted Mortality Rates

Example:You are a primary care practitioner in a

suburb of a medium-sized city. Some of your patients lately have been

older men with cardiac chest pain, andafter appropriate testing you have decidedthat several of them should undergocoronary artery bypass surgery.

There are 2 groups of surgeons and youwant to send your patients to the groupwith lowest mortality rate.


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Direct Comparison of MortalityRate

511000411000Total 45-74

364002020065-74

123001530055-64

3300650045-54

Operative

Mortality

Operative

Death

SurgeriesOperative

Mortality

Operative

Death

Surgeries

Clinic BClinic A

Age group


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Adjusted Rates

Step 1: Calculate age-specific rates in the populationsyou want to compare

5.1%5110004.1%411000Total 45-74

9%3640010%2020065-744%123005%1530055-64

1%33001.2%650045-54

Operative

Mortality

Operative

Death

SurgeriesOperative

Mortality

Operative

Death

Surgeries

Clinic BClinic A

Age group


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Adjusted Rates

Step 2 : Choose a reference population

whose age composition is known.

US 2000 standard population

45-54 years 134,834

55-64 years 87,247

65-74 years 66,037


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Adjusted Rates

Step 3:Calculate Expected deaths inreference population if the Clinic A specificrates were the true rates


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Adjusted Rates

1348.3

Clinic B

(1)*(3)

12584.1

1618.0

Clinic A

(1)*(2)

Expected # of deaths in

ref pop using rates

from

Operative Mortality

66,037

87,247

134,834

No. in ref

pop (1)

5.1%4.1%Total 45-74

9%10%65-74

4%5%55-64

1%1.2%45-54

Clinic B(3)Clinic A(2)

Age group


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Adjusted Rates

Step 4. Add up the total number of deaths expectedin the reference population under each clinic's setof rates.

10781.5

5943.33

3489.88

1348.3

Clinic B

(1)*(3)

12584.1

6603.70

4362.35

1618.0

Clinic A

(1)*(2)

Expected # of deaths in refpop using rates from

Operative Mortality

66,037

87,247

134,834

No. in ref pop

(1)

5.1%4.1%Total 45-74

9%10%65-74

4%5%55-64

1%1.2%45-54

Clinic B(3)Clinic A(2)

Age group


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Adjusted Rates

Step 5 Calculate the age adjustedoperative mortality rate for each studygroup by dividing the sum of expecteddeaths by the total size of the referencepopulation:

Expected # of deaths

Total size of the ref. pop. x 100


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Adjusted Rates

Clinic A: 12,584.1 / 288,118 * 100% = 4.37 / 100surgeries

Clinic B: 10,781.5 / 288,118 * 100% =3.74 / 100surgeriesWhich clinic would you recommendnow?


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Summary

Counts of disease and other health events

are important in epidemiology.

Counts are the basis for disease

surveillance and for allocation of

resources.

However, a count alone is insufficient fordescribing the characteristics of a

population and for determining risk. For

these purposes we use ratios,


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Summary

Ratios and proportions are useful for

describing the characteristics of

populations. Proportions and rates are used for

quantifying morbidityand mortality.

From these proportions we can inferriskamong different groups, detect high-risk

groups, and develop hypotheses about

causes i.e. wh these rou s are at increased risk.


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Summary

The two primary measures of morbidity

are incidence ratesand prevalence.

Incidence rates reflect the occurrence of

new disease in a population;

prevalence reflects the presence of

disease in a population.


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Summary

To quantify the association between

disease occurrence and possible risk

factors or causes, we commonly use two

measures, relative riskand odds ratio.

(these will be discussed in later lessons )


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Summary

Mortality rates have long been thestandard for measuring mortality in apopulation. Recently, years of potentiallife lost and years of potential life lostrates have gained in popularity becausethey focus on premature, and mostly

preventable, mortality. All of these measures are used when we

perform the core epidemiologic taskknown as descriptive epidemiology.

Table 2. shows a summary of the formulas forfr

equently used morbidity measures


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frequently used morbidity measures.


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Table 3 summarizes the formulasof frequently used mortality

measures.

Table 3


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