What is a population?
What is population health?
Epidemiology matters: a new introduction to methodological foundations
Chapter 2
A brief introduction to the graphical notation used in Epidemiology
Matters
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Diseased
Non-diseased
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Set of individuals
100 individuals
20 diseased individuals
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Multiple sets of individuals
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1. Defining a population of interest
2. Dynamic versus stationary populations
3. Summary
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Seven steps
1.
Define the population of interest
2.
Conceptualize and create measures of exposures and health
indicators
3.
Take a sample of the population
4.
Estimate measures of association between exposures and health
indicators of interest
5.
Rigorously evaluate whether the association observed suggests a
causal association
6.
Assess the evidence for causes working together
7.
Assess the extent to which the result matters, is externally valid,
to other populations
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1. Defining a population of interest
2. Dynamic versus stationary populations
3. Summary
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Defining a population of interest
 A population is a collection of individuals, at
moments in time, defined by at least one organizing
characteristic
 The definition of a population has implications for
analysis, interpretation, and generalizability of
results from epidemiologic studies
.
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Two axes of population definitions
1. Populations are defined by eligibility criteria
2. Populations are defined by whether individuals
move in and out of eligibility; i.e., populations can
be dynamic or stationary
.
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What could be eligibility criteria?
1. Geographic area and time period of interest
2. Characteristics of persons, events, or exposures for
which health-related factors are of interest
3. Factors that promote successful study completion
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Population, defined by
geographic space and time
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Dynamic population of Farrlandia
over 1 year
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Dynamic population of Farrlandia
over 1 year
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Dynamic population of Farrlandia
over 1 year
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Examples of populations, defined by
characteristic, event, or exposure
 If we are interested in understanding determinants of neonatal health, the
population of interest is all newborn infants. Therefore, the defining event
for inclusion is date of birth, and eligible individuals for population can be
those within a year of their birth.
 Pregnant women
 Individuals over the age of 65
 Individuals exposed to a natural or human-made disaster
 Individuals of certain races or ethnicities
 Individuals subject to particular laws
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An example
 After the World Trade Center 2001 attack, a study considered
neonatal outcomes among offspring of women in California who
gave birth six months after the attack comparing them to women
who gave birth six months before the attack
 This study therefore defined population of interest defined by
a. Event or exposure = pregnancy
b. Geographic location = California
c. Time period = six months before and after September 11,
2001
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Populations, defined by study considerations
Population of interest may be select group collected
specifically to achieve successful study
 Good responders to our surveys
 Likely to continue to attend the health facility for followup visits
 Individuals who are healthy so that they are unlikely to
die during the course of the study outside scope of
inquiry
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An example
The Women’s Health Initiative is a study that assesses
health benefits and risks of hormone replacement therapy
after menopause. Eligibility is limited to women who had
undergone menopause.
The study excludes
a. Women with a history of certain medical conditions
b. Women who were unlikely to be adherent to study
protocol
c. Women who failed to meet other criteria
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1. Defining a population of interest
2. Dynamic versus stationary populations
3. Summary
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Dynamic vs stationary populations
Dynamic eligibility criteria for the population: allow movement in
and/or out of the population. For example, people may move out of
geographic region and no longer be considered part of the population
of interest.
Stationary eligibility criteria for the population: do not allow
movement in and/or out of the population. For example, population
may include all those born on a particular date only and follow them
forward in time, never losing any nor adding others.
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1. Defining a population of interest
2. Dynamic versus stationary populations
3. Summary
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What is a population?
What is population health?
 Epidemiology is concerned with understanding the health of
populations
 Populations of interest can be defined by geography, space, time or
by characteristics of participants or of events of interest
 Regardless of eligibility criteria, population may be dynamic or
stationary
 Careful definition of the population base from which we conduct
an epidemiologic study underlies many of the core methods in
epidemiology
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Seven steps
1.
Define the population of interest
2.
Conceptualize and create measures of exposures and health
indicators
3.
Take a sample of the population
4.
Estimate measures of association between exposures and health
indicators of interest
5.
Rigorously evaluate whether the association observed suggests a
causal association
6.
Assess the evidence for causes working together
7.
Assess the extent to which the result matters, is externally valid,
to other populations
Epidemiology Matters – Chapter 1
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epidemiologymatters.org
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