Epidemiology quiz

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Populations

David Celentano, ScD, MHS Johns Hopkins University

Key Domains of Epidemiology

n  The study of how disease is distributed in populations and what factors influence or determine this distribution

Epidemiology

n  Assess the burden of disease

n  Assess risk factors for disease

n  Evaluate interventions

n  Make policy

n  Communicate with the public

Principal Functions of Public Health

n  How many people are affected? u  Helps to determine how important the problem is to individuals within a population

and to society u  Allows determination of whether the problem merits the risks and benefits of

intervention

Burden of Disease

n  Assess the burden of disease

n  Assess risk factors for disease

n  Evaluate interventions

n  Make policy

n  Communicate with the public

Principal Functions of Public Health

n  How do the patterns of occurrence of the public health problem vary by person, place, and time? u  Helps to generate hypotheses about the causes of the disease u  Directs research to determine the cause of the disease

Risk Factors for Disease

n  Does exposure A cause outcome B?

n  Research questions are targeted to specific populations

Exposures and Outcomes

B Exposure Outcome

? A

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Populations in Epidemiology

n  Decline in Estimated Glomerular Filtration Rate and Subsequent Risk of End-Stage Renal Disease and Mortality

n  Coresh et al

n  JAMA. 2014;311(24):2518–2531

n  doi:10.1001/jama.2014.6634

Investigation

n  Any sizable aggregate of people who satisfy a particular set of membership criteria

n  In epidemiological studies, we usually describe the membership criteria by three characteristics: 1.   Person: age, sex, race, ethnicity, occupation 2.   Place: country, state, city, place of employment 3.   Time: season, calendar year, life stage

What Is a Population?

n  Define population at risk, exposure of interest, and outcome

n  Does A cause B? u  Exposure: “A” u  Outcome: “B”

n  Who should be studied?

n  How do they relate to those eligible to be studied?

n  How do they relate to those at risk for outcome?

Formulating a Public Health Research Question

n  Target population

n  Source population

n  Study population

Types of Populations in Epidemiology

n  The target population is the group of individuals about whom inferences are to be made

n  Sometimes it is not possible or feasible to enumerate or count the entire target population

Target Population

n  Identify a group of individuals from the target population who can be enumerated for further study

n  This is the source population

Source Population

n  The study population is the group of individuals selected from the source population who participate and are observed in the study

Study Population

n  Eligibility u  May have specific criteria that eliminates some individuals u  For example, free of disease at baseline

n  Feasibility u  Total number of individuals may be too large or cost may be too high

n  Participation u  Some individuals in the source population may decline participation

Why Not Study the Entire Source Population?

n  If it is feasible to enumerate and study each individual in the target population, then:

n  Example: database of all people ≥ 65 years of age and enrolled in Medicare

Sometimes Target = Source = Study

Internal and external validity

n  Degree to which a study is free from bias or systematic error

n  Soundness of study design, conduct, and analysis in answering the question that it posed for the study participants

n  Prerequisite for external validity

Internal Validity: Definition

20 Source: Porta M, ed. (2008). A Dictionary of Epidemiology, 5th edition. Oxford University Press.

Internal Validity

n  A study has internal validity if the inferences from the study population reflect the inferences that would be observed in the target population

Internal Validity

n  Depends on knowledge of study methods: u  Selection of study participants u  Data collection methods u  Conduct of analyses

n  Depends on knowledge of subject matter: u  Application of correct dose of drugs in a clinical trial u  Choice of window of exposure in a longitudinal study u  Planning of valid intervals of measurement u  Planning of procedures for outcomes ascertainment

Internal Validity

n  Vague definition of the target population

n  Inability to define the source population

n  Problems with enrolling the study population

Barriers to Internal Validity

n  “Generalizability”

n  “Degree to which the results of the study may apply, be relevant, or be generalized to populations or groups that did not participate in the study”

External Validity

25 Source: Porta M, ed. (2008). A Dictionary of Epidemiology, 5th edition. Oxford University Press.

External Validity: Assessing Whether Internally Valid Inferences Apply to other Target Populations

n  Depends on knowledge of methods u  Composition of the study population

•  Inclusion and exclusion criteria

n  Depends on knowledge of subject matter u  Are circumstances of exposure similar? u  Are susceptibility factors similar? u  Are other related factors similar? u  Would the same results be expected:

•  For men and women •  Across disease stages •  For other racial or ethnic groups

External Validity

n  There is no statistical test or automated method to ensure that inferences are applicable to other populations

n  Ultimately, a matter of informed judgment

n  However, inferences should have internal validity!

External Validity

n  A study of smoking and lung cancer in men u  Can we generalize to women?

n  A study of risk factors for coronary artery disease among middle-aged American men u  Can we generalize to middle-aged European men?

n  A study of risk factors for coronary artery disease among middle-aged American men u  Can we generalize to women?

External Validity

Populations at Risk

Section B

!  Able to experience the endpoint of interest during the follow-up period

What Does it Mean to be at Risk?

!  Biologic !  Susceptibility !  Target organ

!  Methodologic !  Fulfill study criteria of population !  Remain in study under observation !  If first occurrence endpoint: no prior history

Characteristics of Risk

Challenges in defining the population at risk

!  Risk for pregnancy?

At-Risk Status Can Change across Lifespan

Age-Adjusted Uterine Cancer Incidence Rates, Corrected and Uncorrected by Hysterectomy Status, Alameda County

Source: Lyon and Gardner. 6

Individuals and populations in time

Populations Can be Dynamic or Fixed

Population Characteristic Example Dynamic Membership is transient and

defined by being in or out of a "state,” in a certain place, during a certain time period

•  Residents of the City of Baltimore in 2013

•  Inpatients in Johns Hopkins Hospital in 2013

Fixed Membership is permanent and defined by an event, in a certain place, during a certain time period

•  Atomic bomb survivors in Hiroshima at the end of World War II

!  What do the circles represent?

At-Risk Individuals over Time

!  What do the circles represent? !  Events

At-Risk Individuals over Time

!  What do the circles represent? !  Events

!  What do the bars represent? what do the arrows represent?

At-Risk Individuals over Time

!  What do the circles represent? !  Events

!  What do the bars represent? what do the arrows represent? !  Censoring

At-Risk Individuals over Time

!  Individuals are censored if: !  They did not experience the event of interest during follow-up up to a specific time

and !  The occurrence of the event after a specific time is uncertain because they are no

longer under follow-up

Censored Individuals

!  Death (when not the endpoint of interest)

!  Withdrawal (study drop-out)

!  Administrative censoring (end of study)

Reasons for Censoring

!  Closed population !  Individuals do not enter the population over time !  Individuals do not exit the population unless they have the outcome of interest or

end of the study

!  Open population !  Individuals may enter the population at different times !  Individuals may exit for reasons other than the outcome of interest

Open and Closed Populations

!  No censoring in a closed population !  Participants enter at the same time !  Participants leave only for endpoint

or end of study

Closed Populations

Open and Closed Populations

!  Closed populations !  Individuals on a

flight to Baltimore

!  Patients admitted to the ICU at JHH on a specific day

!  Open populations !  Population of

Baltimore in 2013

!  Students in doctoral programs at JHSPH

Populations Moving through Time

The material in this video is subject to the copyright of the owners of the material and is being provided for educational purposes under

rules of fair use for registered students in this course only. No additional copies of the copyrighted work may be made or distributed.

Person-Years

Section C

The Concept of

Person-Years

Challenges to using person-years

0 1 2 3 4 5

The Concept of Person-Years

period of highest risk for disease

0 1 2 3 4 5

The Concept of Person-Years

period of highest risk for disease

0 1 2 3 4 5

The Concept of Person-Years

period of highest risk for disease

0 1 2 3 4 5

The Concept of Person-Years

period of highest risk for disease

!  Are 20 smokers followed for one year equivalent to one smoker followed for 20 years?

Assumption of Constant Risk

Measuring Disease in Populations: Case Definitions

Section D

!  Ideal case definitions are clear, unambiguous, and easy to use

!  Classification system for cases and disease outcomes: !  Confirmed !  Probable !  Suspected

Types of Case Definitions

!  Onset !  Sudden !  Prolonged

!  Duration !  Acute (short) !  Chronic (long)

!  Recurrence !  Single occurrence !  Recurrent

Characteristics Important for Case Definitions

Establishing a Case Definition

Source: New Engl J Med. 2003; 348: 1986-94. 4

Prevalence of Rheumatic Heart Disease

Summary: Populations

The group studied

The group from whom the

study population is drawn

The group to whom

inferences will be made

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