Introduction to Epidemiology

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Introduction to Epidemiology

• UT-H HSC
• Jan Risser

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Organization of class

• Readings
• Homework Homework               20
• Exams Mid Term Exam 30
• Final Exam                 30
• Project Phase I  5
• Phase II                12
• Presentation            3

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Organization of Class

• Brownsville
• Masters of Public Health Nursing
• Texas Department of Health
• Certificate in Public Health

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Organization of Class

• ITV days Thursday from 6 to 9
• 8/31
• 9/21
• 10/5
• 10/26
• 11/16
• 12/7

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Organization of Classes

• Brownsville days NOT broadcasted
• 8/24
• 9/7
• 9/28
• 10/19
• 11/9

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Organization of Classes

• Midterm and Final
• Brownsville to be proctored
• TDH ? to be proctored
• Online resources
• http//www.bmj.com/collections/epidem/
• http//www.ucc.ie/acad//phbh/lectures/epidemiology
  /
• http//www.sph.uth.tmc.edu/courses/epi/Jrisser/PH2
  610D/
• http//www.sph.uth.tmc.edu/courses
• then click on epidemiology
• then click on the outreach education

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• What is epidemiology?

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• Epidemiology involves a way of thinking
• Training the mind to think critically
• Epidemiology is a systematic way of reasoning to
  solve a problem

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Epidemiology

• Assumption
• Disease does not occur randomly
• Disease has identifiable causes
• which can be altered
• and therefore prevent disease from developing
• Source Hennekens CH and Buring JE. Epidemiology
  in Medicine.
• Boston Little Brown Co. 1987

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Definition of Epidemiology

• The study of the distribution and determinants
  of health-related states or events in specified
  population, and the application of this study to
  control of health problems.
• source Last (ed.) Dictionary of Epidemiology,
  1995

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Definition of Epidemiology

• Key components that encompass all
• epidemiologic principles and methods
• Frequency
• Distribution
• Determinants

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Definition of Epidemiology

• Frequency
• quantification of the presence of disease
• prerequisite for any systematic investigation of
  the patterns of occurrence

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Definition of Epidemiology

• Frequency in populations
• Rates and proportions
• Averages
• Dynamics demography, environment, lifestyle

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Definition of Epidemiology

• Distribution
• analysis by time, place, and person
• who, when, where
• how many - counts, ratios, differences

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Steps of epidemiologic research

• The research question
• describe EXPOSURE and OUTCOME
• in terms of
• Person
• Place
• Time

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Steps of epidemiologic research

• Exposure exposure to dirty needles
• Outcome risk factor for HIV
• Person among IVDUs
• Place in Houston, Texas
• Time between 1/2000 and 12/2001

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Steps of epidemiologic research

• To describe and measure the distribution of
  disease in the population
• Define a case / count cases
• Describe who has the disease
• Describe when and where the disease occurred
• Finally, compare outcomes among groups

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Defining case

• Case an individual with the outcome
• Criteria
• clinical vs. research
• Definition must be clear, concise to afford
  accurate comparison

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Defining case

• Definition may be restrictive or broad depending
  on the scope of the study
• Detection and diagnosismay be difficult in
  latent or subclinical disease, so diagnostic
  criteria may be included in case definition
  criteria

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Defining case

• Natural history
• the course (stages) of disease from exposure to
  ultimate resolution without intervention
• critical in detection, diagnosis, establishment
  of causality, and prevention/control

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Defining case

• Natural course of disease
• Exposure ? onset ? symptoms ? dx ? outcome

Incubation period
Recovery Death Chronic disease
Clinical Stage
Subclinical Stage
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Estimating disease frequency

• When detection is not available or not feasible,
  subclinical cases pose biggest threat for
  establishing reliable estimates of disease
  occurrence underestimation of truth

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Example

• Outbreak investigation
• Food-borne disease associated with church picnic
• illness 6 to 24 hours after a church picnic
• vomiting diarrhea
• chills nausea
• fever

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Example

• Case definition
• vomiting (more than 2 times in 24 hours) 6 or
  more hours after picnic
• nausea, chills and
• diarrhea (more than 3 bowel movements in 24
  hours) 6 or more hours after picnic

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Example

• Who, When, Where
• How many attended picnic
• How many got sick
• When and what the people ate and drank

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Example

• 80 attended
• 55 got sick within 6 to 10 hours
• Ill
• potato salad 55/55
• ham sandwiches 10/55
• hamburgers 4/55
• chicken salad 2/55

This can help us determine what the sick
individuals ate, but can it help us identify the
probable cause of the illness? What is missing?
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Example

• 80 attended
• 55 got sick within 6 to 10 hours
• Ill Not Ill
• potato salad 55/55 25/25
• ham sandwiches 47/55 8/25
• hamburgers 4/55 13/25
• chicken salad 2/55 2/25

Would percentages or rates be easier to compare?
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Example

• 80 attended
• 55 got sick within 6 to 10 hours
• Ill Not Ill
• potato salad 55/55 100 25/25 100
• ham sandwiches 47/55 85 8/25 32
• hamburgers 4/55 7 13/25 52
• chicken salad 2/55 4 2/25 8

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Measures of disease frequency

• Counts
• Measures of morbidity
• Prevalence
• Cumulative incidence
• Incidence rate
• Measures of mortality
• Mortality rate
• Standardized mortality rate (SMR)

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Counts

• Prerequisite for epidemiologic investigation
• Simplest measure of disease frequency
• Frequency of affected individuals
• Useful for planning adequacy of health care
  allocation at a particular level (local,
  regional, national, international)

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Relating disease frequency and exposure

• RATIO
• male to female, Houston to Dallas
• PROPORTION
• prevalence and cumulative incidence
• RATE
• incidence density events occurring over time

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Ratio

• Value obtained by dividing one quantity by
  another
• Numerator not necessarily in denominator
• Ratios used to assess the relative importance of
  an attribute between population sub-groups

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Ratio

• A fraction with no specified relationship between
  numerator and denominator
• Range 0 to ?
• A/B
• Examples
• sex ratio (MF)
• odds ratio

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Proportion

• Type of ratio
• Numerator included in denominator
• May be expressed as percentage
• Range 0 to 1
• A/(AB)
• Example
• Prevalence (always a proportion)
• Cumulative Incidence (proportion at risk)

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Proportions

• Ratio where the numerator is always part of the
  denominator
• Common proportions used are percentages
• Proportions assess the relative importance of an
  attribute in the population
• Prevalence -
• proportion of population with the attribute

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Rate

• A special type of proportion
• Unit of time in denominator
• Best for comparison of disease frequencies among
  subsets of population
• Common denominator population from which cases
  came

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Rate

• A change in one quantity per unit change in
  another (usually time)
• new cases of disease over time
• 6 new cases of liver cancer per year
• new cases of disease over changing biological
  parameter
• 4 new cases of myocardial infarction per unit
  increase in serum cholesterol

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Rate

• The number of events in a specific time period
  divided by the population at risk in the
  specified time period

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Measures of Disease Frequency

• Prevalence
• Incidence
• Incidence Density
• These allow us to summarize a groups experience
  with a particular health event with a single
  expression.

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Prevalence (a proportion)

• the proportion of the population at a given time
  that have the factor of interest
• Prevalence of an exposure
• what proportion of this class grew up drinking
  fluoridated water
• Prevalence of outcome
• what proportion of this class have dental caries

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Prevalence

• Numerator
• all those with the attribute at a particular time
• Denominator
• the population during that same time period
• Choice of denominator may be difficult

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Prevalence

• In 1997 there were 1,854 cases of syphilis in
  Harris County
• What should be used for the denominator?
• During 6 months, 3 states reported 55 cases of a
  new disease among menstruating women
• What should be used for the denominator?

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Prevalence

• Types
• Period prevalence proportion of all cases at a
  specific point in time
• Point prevalence proportion of all cases during
  a period of time
• Frequency of disease at autopsy all cases among
  those autopsied
• Birth defect rate number of new births with
  abnormality among all live births

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Prevalence

• Utility
• Describe health burden of a population
• Status of disease in a population
• Estimate the frequency of exposure
• Project health care needs of affected individuals

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Prevalence

• Limitations
• All cases with a disease at a given point in time
  are a product of
• Factors influencing development of disease
• Factors influencing survival (duration, severity)
• Difficulty in establishing antecedent events

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Incidence

• Rate of development of disease during a given
  period of time
• Three key elements
• Only new cases included in numerator
• Total population at risk in the denominator
• Time element period over which new cases
  developed

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Cumulative Incidence

• the probability (risk) of an individual
  developing the disease (outcome) during a
  specific period of time

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Incidence density

• the probability (risk) of an individual
  developing the disease (outcome) during a
  specific period of time, using total person-time
  as the denominator.
• One subject followed one year contributes one
  person-year (PY)

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Calculating person-time
x
x
ID 2/17 11.8/100 person-years of observation
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Incidence, Prevalence
Onset
A
B
C
D
E
F
1994
1986
1988
1990
1992
What was prevalence of disease in 1992? What is
risk of developing disease within 2 years?
1 case (A) / 4 subjects) 25
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Incidence, Prevalence
Onset
A
B
C
D
E
F
4
6
8
0
2
Incidence within 2 years
1/6 17
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Why study rates / ratios

• Epidemiology is the study of the distribution and
  determinants of disease in human population
• We describe distribution by describing
  Person Place Time
• We do this to try to understand WHY disease
  development is not random

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Ways to present proportions and rates

• Crude rates
• Specific rates
• Adjusted rates
• note I am including prevalence, incidence and
  incidence density with the generic term rates

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Descriptive Epidemiology

• Calculating rates requires that you COUNT the
  numbers of individuals experiencing the health
  event (numerator) and you ESTIMATE the population
  at risk (denominator)
• Definition and ascertainment of CASES
• CASES individuals with the OUTCOME of interest

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Descriptive Epidemiology

• Outcome of interest may be
• alive or dead
• infected or not-infected
• pregnant or not-pregnant
• with birth defect or without

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CRUDE RATE

• number of events in a specified time period
• population at risk in that time period
• This is the overall measurement of the event
• does not take into consideration PERSON
• such as age, gender, race/ethnicity, residence,
  or socioeconomic status

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CRUDE RATES

• Birth Rate to project population changes
  affected by the and age composition of women of
  childbearing age
• of live births / population
• Fertility Rate for comparisons of fertility
  among age, racial, and socioeconomic groups
• of live births / of women between 15-44

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Crude Rates

• Infant mortality rate for international
  comparisons a high rate indicates unmet health
  needs and poor environmental conditions
• infant deaths (lt1year) / of live births

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CRUDE RATES

• Fetal Death Rate - to estimate the risk of death
  of the fetus associated with the stages of
  gestation
• Neonatal Mortality Rate reflects events
  happening after birth, primarily
• 1. congenital malformations
• 2. prematurity (birth before 28 weeks gestation)
• 3. low birth weight (weight less than 2,500 g)
• .

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Crude Rates

• Perinatal Mortality Rate reflects events that
  occur during pregnancy and after birth it
  combines mortality during the prenatal and
  postnatal periods

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CRUDE RATES

• Maternal Mortality Rate reflects health care
  access and socioeconomic factors it includes
  maternal deaths resulting from causes associated
  with pregnancy and during and after childbirth

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Crude Rates

• Cause Specific Mortality Rate - reflects the
  burden one particular disease has on total
  mortality
• Accidents
• HIV infection
• Cancer
• Suicide

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Strata Specific Rates

• If we wanted to compare mortality or morbidity
  among specific sub-groups of the population
• we would need to use stratified rates
• so that we take into account the differences in
  the proportion of the population each subgroup
  represents.
• Strata specific rates are better indicators of
  risk than crude rates

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Strata Specific Rates

• Rather than the whole population present the
  event rates by categories.
• The outcome is grouped by a particular trait
• The CRUDE RATE may hide information Without
  examining the strata specific rates you would
  not be able to appreciate that risk of an adverse
  event is different in specific segments of the
  population

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Strata Specific Rates

• Characteristics of PERSON
• Age
• Sex
• Race/ethnicity
• Marital status
• Nativity and migration
• Religion
• Socioeconomic Status

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Category-specific rates

• Stratifying on PERSON characteristics provides
  better description of who is at risk
• plan interventions
• prescribe program needs

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Strata Specific Rates

• Describe who is at risk
• plan interventions
• prescribe program needs

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Age-Specific death rates in the US
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Mortality of Tuberculosis by Age
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Strata Specific Rates

• Characteristics of PLACE
• International
• Geographic Variations
• Urban/Rural differences
• Localized occurrence of disease

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Category-specific rates

• Utility of analysis by PLACE
• Serve as an aid in establishing priorities for
  allocation of resources
• Serve as an aid in developing policy for
  community health and social programs
• Identifies high- and low-risk areas
• Provide a reliable method of presenting data to
  concerned groups

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Geographic Stratification Heart Disease
Mortality, 1998
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Geographic and Race/Ethnicity Stratification
Heart Disease Mortality, 1998
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Lyme Disease

• most common vectorborne disease in the US
• caused by a spirochete
• deer tick - the primary vector
• geographic distribution of Lyme disease
  correlates with the natural distribution of the
  deer tick

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Category-specific rates

• Analysis based on characteristics of TIME
• Cyclic fluctuations
• Point epidemics
• Secular time trends
• Cohort effects
• Clustering

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Strata Specific Rates

• Describe where risk is greatest
• describe high- and low-risk areas
• Provide a "community diagnosis" for further
  investigation
• Provide a basis for determining the health status
  of the population

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Strata Specific Rates

• Provide a reliable method of presenting data to
  concerned groups
• Serve as an aid in establishing priorities for
  allocation of resources
• Serve as an aid in developing polity for
  community health and social programs

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Strata Specific Rates

• Characteristics of TIME
• Cyclic fluctuations
• Point epidemics
• Secular time trends
• Cohort effects
• Clustering

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Chlamydia TIME number of cases and rates
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Chlamydia Person Age and Gender
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Gonorrhea Person Age and Gender
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Chlamydia PLACEPercent Infected at Screening
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Syphilis PERSON Race/ethnicity and gender
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(No Transcript)
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Category-specific rates

• Limitations
• Category-specific rates may be confusing when
  stratifying on multiple variables such as age,
  race, gender
• Tables become incomprehensible
• Use of adjusted rates preferred in these
  situations

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Adjusted rates

• a summary rate that is re-calculated to correct
  for (or adjust for) some additional factors
• called confounding factors
• such as age, gender, ethnicity, SES, numbers of
  cigarettes smoked, etc.

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Adjusted Rates

• Be aware that you might be hiding or distorting
  information if you average events over strata
• The distorting variable may be time, age,
  race/ethnicity or any other variable that effects
  the outcome
• With crude rates you are assuming that the events
  occurred evenly spaced among the possible sets

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Crude rates

• Indicate the actual experience of the population.
  
• They are the real number of events divided by the
  real population.
• The crude rate is affected by the strata specific
  rates and by the strata-specific structure of the
  population
• The crude rate may be modified (confounded) by
  person factors in the population

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Adjusted rates

• Two methods
• Direct
• Indirect
• Difference is in the information available

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Standardized (adjusted) rates

• Used to minimize distortion when comparing crude
  rates
• Allows comparison of rates between populations
  that may differ by factors that could influence
  crude rate (e.g., age, gender, ethnicity)

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Direct Adjustment of Rates

• If age-specific death rates in the population to
  be standardized are known and a suitable standard
  population is available

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Age-adjusted rates /crude rates
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Adjusted rates

• Age-adjusted rates are used
• to compare population rates
• after eliminating the effects of differences in
  the distribution of age in the compared
  populations

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Adjusted rates

• An adjusted rate is a fictitious number
• It expresses what the rate would be in a standard
  population
• which is usually not a real population
• if that population had experienced the same
  stratum-specific rates as the real population

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Adjusted rates

• The actual value of an age-adjusted rate is
  meaningless because it depends on the choice of
  the standard population
• It is only meaningful in comparison to other
  rates which have been adjusted by the same method
  and the same standard population

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Adjusted rates

• It is useless to compare rates that have not been
  adjusted using the same standard population.

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Proportional Mortality Ratio

• Number of deaths due to a specific disease
  proportionate to all deaths in a population
• Not a measure of the risk of dying from a
  particular disease
• Relative importance of a particular disease
• Useful for health care planning and indicating
  areas for further study