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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/Prognosis8/14/2019 Measures of Disease Frequency 0903_gaohongcai(2)
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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.
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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.
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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
8/14/2019 Measures of Disease Frequency 0903_gaohongcai(2)
84/86
84
8/14/2019 Measures of Disease Frequency 0903_gaohongcai(2)
85/86
8/14/2019 Measures of Disease Frequency 0903_gaohongcai(2)
86/86
Question?