Epidemiology Master Level Quiz

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StudyDesignsCase-ControlStudiesandOtherNestedDesigns.pdf

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Study Designs: Case-Control Studies and Other Nested Designs

David Celentano, ScD, MHS Johns Hopkins University

Step 1: Is There an Association?

Disease or other

health outcome Exposure

Can use different study designs to address this research question

Main Types of Epidemiologic Study Designs

Study type Characteristics

Experimental • Studies prevention and treatment of disease • Investigator actively manipulates which groups receive the study agent

Observational • Studies causes, prevention and treatment for diseases • Investigator watches as natures takes its course

Cohort • Examines multiple health effects of an exposure • Subjects defined by exposure levels and follow for disease occurrence

Case-control • Typically examines multiple exposures in relation to a disease • Subjects are defined as cases and controls and exposure histories compared

Cross-sectional • Examine relationship between exposure and disease prevalence in a defined population at one point in time

Ecological • Examines relationship between exposure and disease with population-level data rather than individual data

Example of Case-Control Study

Example of Case- Control Study

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Design of Case-Control Studies

!  When the disease is rare (low prevalence)

!  Observing a series of cases, it is difficult to interpret what would be occurring in non- cases !  Hence, we need a comparison—or “control”—group

!  Comparison is an essential component of any epidemiologic investigation

Typical Uses of Case-Control Design

Design of a Case-Control Study

Case-Control Study

Cases (with disease)

Controls (without disease)

Were exposed a b

Were not exposed c d

Totals a + c b + d

Proportions exposed a

a + c b

b + d

First, select:

Case-Control Study

Cases (with disease)

Controls (without disease)

Were exposed a b

Were not exposed c d

Totals a + c b + d

Proportions exposed a

a + c b

b + d

Then,

measure

past

exposure

Case-Control Study

Eurosurveillance.org: June, 2013

Design of a Case-Control Study

Case-Control Study

Cases (with disease)

Controls (without disease)

Were exposed a b

Were not exposed c d

Totals a + c b + d

Proportions exposed a

a + c b

b + d

First, select:

Then,

measure

past

exposure

Cohort Study

Disease

develops

Disease

does not

develop Totals Incidence

Were exposed

a b a + b a

a +b

Were not exposed

c d c + d c

c + d

Then, follow to see whether:

First,

select:

a a + b

= Incidence in exposed c

c + d = Incidence in non-exposed

Case-Control Study

CHD Controls

Smoke cigarettes 112 176

Do not smoke cigarettes 88 224

Total 200 400

% smoking cigarettes 56.0% 44.0%

In a Case-Control Study, We Can Study Multiple Exposures

Where Do the Cases Come From?

Control Selection and the Interpretation of Results in Case-Control Studies

Section B

!  Eligible to become cases if develop disease

!  Similar to cases on key factors

!  Representative of “source” population

Control Selection

!  Hospital/clinic based !  Convenient !  Difficult in ensuring that exposure reflects underlying reference population

!  Population/community controls !  Often less convenient/more expensive !  Less of above concerns

Types of Controls

Some Types of Non-Hospital Controls

!  Probability sample of total population

!  Neighborhood !  Walk (door to door) !  Random digit dialing !  Letter carrier route

!  Best friend or associate

!  Spouse or sibling

!  Birth certificate match !  For childhood diseases

!  Classmate !  For childhood diseases

Selection of Controls: MacMahon's Case-Control Study of Pancreatic Cancer

!  Included !  All other patients who were under

the care of the same physician in the same hospital at the time of the interview of a patient with pancreatic cancer

!  Excluded !  Patients with pancreatic or

hepatobiliary tract diseases known to be associated with smoking or alcohol consumption

!  Patients with cardiovascular disease, diabetes, respiratory or bladder cancer, and peptic ulcer

!  Non-Whites; older than 79 years

Estimated Relative Risk of Pancreatic Cancer by Sex and Coffee Drinking Habits

Source: (1981). MacMahon et al. 8

0 1–2 3–4 >5

Men 1.0 2.6 2.3 2.6

Women 1.0 1.6 3.3 3.1

Both sexes 1.0 2.1 2.8 3.2

Coffee drinking (cups/day)

Estimated Relative Risks of Pancreatic Cancer for Use of Coffee and Cigarettes

Source: (1981). MacMahon et al. 9

Cigarette smoking 0 1–2 >3 Total

Never 1.0 2.1 3.1 1.0

Ex-smokers 1.3 4.0 3.0 1.3

Current smokers 1.2 2.2 4.6 1.2

Total 1.0 1.8 2.7

Coffee drinking (cups/day)

Interpreting the Results of a Case-Control Study of Coffee and Pancreatic Cancer

Matching

!  The process of selecting controls so that they are similar to the cases in regard to certain characteristics such as age, sex, and race

Matching

!  Group matching !  Frequency matching, stratification

!  Individual matching !  Matched pairs

Matching in Case-Control Studies

Multiple controls

A.  Controls from the same source; controls of a similar type !  For example, two hospital controls per case

B.  Controls from different sources !  For example, hospital and neighborhood controls

!  How does this help? !  Increases the statistical probability of showing a true difference if there is one !  Improves generalizability of findings

Types of Multiple Controls Used in Case-Control Studies

Study Groups in Gold’s Study of Brain Tumors in Children

Source: (1979). Gold et al. 17

Study Groups in Gold’s Study of Brain Tumors in Children

Source: (1979). Gold et al. 18

Study Groups in Gold’s Study of Brain Tumors in Children

Source: (1979). Gold et al. 19

!  Limitations in human ability to recall

!  Recall bias: cases may remember their exposure more than controls do

!  Cases may also attribute exposures

Problems of Recall in Case-Control Studies

!  Matching on many variables may make it difficult to find an appropriate control

!  We cannot explore the possible association of the disease with any variable on which the cases and controls have been matched !  Therefore, you need to be cautious about what you choose to match

Problems with Matching

!  Example: A study of maternal infections during pregnancy and congenital malformation

An Artificial Association Resulting from Recall Bias

Assume: Cases (congenital malformations)

Controls (no malformations)

True incidence of infection 15% 15%

Percent of infections recalled 90% 10%

Infection rate as ascertained by interview 13.5% 1.5%

Nested Case-Control Studies

Section C

Design of a Nested Case-Control Study

Case-Control Study

Nested Case Control Example

Nested Case- Control within a Cohort Study

1.  Possibility of recall bias is eliminated because data on exposures are obtained before disease develops

2.  Exposure data are more likely to represent the pre-illness state because they are obtained years before clinical illness is diagnosed

3.  Costs are reduced compared to those of a prospective study because laboratory tests need to be done only on specimens from subjects who are later chosen as cases or as controls

Advantages of Nested Case-Control Studies

1.  Described the structure and design elements of case-control studies

2.  Described the selection of cases and controls—and the process of matching

3.  Discussed potential sources of bias and distinguished between nested and non-nested designs

Summary

Main Types of Epidemiologic Study Designs

Study type Characteristics

Experimental • Studies prevention and treatment of disease • Investigator actively manipulates which groups receive the study agent

Observational • Studies causes, prevention and treatment for diseases • Investigator watches as natures takes its course

Cohort • Examines multiple health effects of an exposure • Subjects defined by exposure levels and follow for disease occurrence

Case-control • Typically examines multiple exposures in relation to a disease • Subjects are defined as cases and controls and exposure histories compared

Cross-sectional • Examine relationship between exposure and disease prevalence in a defined population at one point in time

Ecological • Examines relationship between exposure and disease with population-level data rather than individual data

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