What is a population? What is population health?

Epidemiology matters: a new introduction to methodological foundations

Chapter 2

Epidemiology Matters – Chapter 2 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 individuals20 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 criteria2. 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 interest2. Characteristics of persons, events, or exposures for

which health-related factors are of interest3. Factors that promote successful study completion

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Population, defined bygeographic space and time

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Dynamic population of Farrlandiaover 1 year

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Dynamic population of Farrlandiaover 1 year

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Dynamic population of Farrlandiaover 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 follow-

up 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 25

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