Student PowerPoint Presentation: Chapter 6 and 7

Objectives: The presentation assignment has several goals. It requires students to apply concepts of study designs, ecology, cross-sectional and case control. Apply and differentiate cohort studies from other epidemiologic study designs able to be used in any Health Care Facility. 

Format and Guidelines: The student will create a Power Point Presentation from Chapter 6 and 7 of the Textbook and the Article related to Week 3 (Choose your desire topic form these chapters). The Presentation should have a minimum of 12 slides, including Title Page, Introduction, Conclusion, and References. 

The student must use other textbooks, research papers, and articles as references (minimum 3).

 

EACH PAPER SHOULD INCLUDE THE FOLLOWING:

1. Title Page: Topic Name, Student Name

2. Introduction: Provide a brief synopsis of the meaning (not a description) of the topic you choose, in your own words

3. Content Body: Progress your theme, provide Material, illustrations and  Diagram to explain, describe and clarify the Topic you choose.

4. Conclusion: Briefly summarize your thoughts & conclusion to your critique of the articles and Chapter you read. 

5. References: The student must use other textbooks, research papers, and articles as references (minimum 3).

ASSIGNMENT DUE DATE:

The assignment is to be electronically posted no later than noon on Saturday, January 25, 2020.

Chapter6

Study Designs: Ecologic,

Cross-Sectional, Case-

Control

Learning Objectives

• Define the basic differences between
observational and experimental epidemiology

• Identify an epidemiologic study design by its
description

• List the main characteristics, advantages, and
disadvantages of ecologic, cross-sectional, and
case-control studies

• Describe sample designs used in epidemiologic
research

• Calculate and interpret an odds ratio

How Study Designs Differ

• Number of observations made

• Directionality of exposure

• Data collection methods

• Timing of data collection

• Unit of observation

• Availability of subjects

Observational vs. Experimental

Approaches

• Manipulation of study factor

– Was exposure of interest controlled by

investigator?

• Randomization of study subjects

– Was there use of a random process to

determine exposure of study subjects?

Typology of Epidemiologic

Research

Overview of Study Designs

• Experimental studies

• Quasi-experimental studies

• Observational studies

– Descriptive studies: cross-sectional

surveys

– Analytic studies: many ecologic studies,

case-control studies, cohort studies

The 2 by 2 Table Represents the

Association Between Exposure and

Disease Status

Ecologic

Studies

• The unit of analysis is the group, not the

individual.

• They can be used for generating

hypotheses.

• The level of exposure for each individual

in the unit being studied is unknown.

• Generally makes use of secondary data.

• Advantageous with cost and duration.

Types of Ecologic Studies

• Ecologic comparison study—involves an

assessment of the correlation between

exposure rates and

disease

rates among

different groups over the same time period.

• Ecologic trend study—involves correlation of

changes in exposure with changes in disease

within the same community, country, or other

aggregate unit.

Example of an Ecologic

Correlation

• The association between breast

cancer and dietary fat for 39

countries.

• High intakes of dietary fats

associated with high rates of breast

cancer mortality.

Examples of Questions Investigated

by Ecologic Studies

• Is the ranking of cities by air pollution levels

associated with the ranking of cities by mortality

from cardiovascular disease, adjusting for

differences in average age, percent of the

population below poverty level, and

occupational structure?

• What are long-term trends (1950-1995) for

mortality from the major cancers in the US,

Canada, and Mexico?

Applications of Ecologic

Approach

• The effect of fluoridation of the water

supply on hip fractures

• The association of naturally occurring

fluoride levels and cancer incidence rates

• The relationship between neighborhood or

local area social characteristics and health

outcomes

The Ecologic Fallacy:

Definition

• Observations made at the group level may

not represent the exposure-disease

relationship

at the individual level.

• The ecologic fallacy occurs when incorrect

inferences about the individual are made

from group level data.

Implications of the Ecologic

Fallacy

• The conclusions obtained from an

ecologic study may be the reverse of

those from a study that collects data on

individual subjects.

The Ecologic Study: Example

• An ecologic study examines 10 individuals who

go into the sun.

• The study finds that 7 persons (70%) have

sunburned foreheads although 6 persons (60%)

wore hats.

• The expected number of sunburned foreheads

is 4 (the number who did not wear hats).

• The media report that wearing hats will not

protect you from sunburn.

What the Individual Data Show

Individual Data (cont’d)

• From the individual data, one observes

that 100% of persons (4) who did not wear

hats were sunburned.

• Among persons who wore hats (6), only

50% were sunburned.

• This conclusion reverses the conclusion

from the ecologic data, i.e., that wearing

hats affords little protection from sunburn.

Ecologic Studies: Advantages

and Disadvantages

• Advantages

– Quick, simple, inexpensive

– Good approach for generating hypotheses

when a disease is of unknown etiology

• Disadvantages

– Ecological fallacy

– Imprecise measurement of exposure and

disease

Cross-Sectional Study

• Also termed prevalence study

• Exposure and disease measures obtained

at the individual level.

• Single period of observation

• Exposure and disease histories are

collected simultaneously.

• Both probability and non-probability

sampling is used.

Cross-Sectional Study:

Examples

• Surveys of smokeless tobacco use among
high school students

• Prevalence surveys of the number of
vasectomies performed

• Prevalence surveys of cigarette smoking
among Cambodian Americans in Long
Beach, California

Uses of Cross-Sectional

Studies

• Hypothesis generation

• Intervention planning

• Planning health services and

administering medical care facilities

• Estimation of the magnitude and

distribution of a health problem

• Examine trends in disease or risk factors

that can vary over time

Limitations of Cross-Sectional

Studies

• Limited usefulness for inferring disease
etiology

• Do not provide incidence data

• Cannot study low prevalence diseases

• Cannot determine temporality of exposure
and disease

Overview of Case-Control

Studies

• In a case-control study with two groups, one

group has the disease of interest (cases) and a

comparable group is free from the disease

(controls).

• The case-control study identifies possible

causes of disease by finding out how the two

groups differ with respect to exposure to some

factor.

Characteristics of the Case-

Control Study

• A single point of observation

• Unit of observation and the unit of analysis

are the individual

• Exposure is determined retrospectively

• Does not directly provide incidence data

• Data collection typically involves a

combination of both primary and

secondary sources.

Selection of Cases

• Two tasks are involved in case

selection:

–Defining a case conceptually

–Identifying a case operationally

Sources of Cases

• Need to define a case conceptually

• Ideally, identify and enroll all incident
cases in a defined population in a
specified time period

• A tumor registry or vital statistics bureau
may provide a complete listing of all cases

• Medical facilities also may be a source of
cases, but not always incident cases

Selection of

Controls

• The ideal controls should have the same

characteristics as the cases (except for the

exposure of interest).

• If the controls were equal to the cases in all

respects other than disease and the

hypothesized risk factor, one would be in a

stronger position to ascribe differences in

disease status to the exposure of interest.

Sources of Controls
• Population-based controls–Obtain a list that

contains names and addresses of most

residents in the same geographic area as the

cases.

– A driver’s license list would include most

people between the ages of 16 and 65.

– Tax lists, voting lists, and telephone

directories

– Patients from the same hospital as the cases

– Relatives of cases

Measures of Association Used

in

Case-Control Studies

Disease Status

Yes (Cases)

No (Controls)

Yes

A

B

E
x

p
o

s
u

re

S
ta

tu
s

No

C

D

A+C B+D

Odds A/C B/D

Odds Ratio AD/BC

Case-Control Studies

Sample Calculation

• On the association between chili pepper

consumption and gastric cancer risk: a

population-based case-control study

conducted in Mexico City

• Source: Lopez-Carillo, et al. Am J

Epidemiol. 1994;139:263-71.

Sample Calculation (cont’d)

Chili Pepper Consumption Cases of Gastric

Cancer

Controls

Yes A = 204 B = 552

No C = 9 D = 145

The OR (unadjusted for age and sex) is:

AD = (204)(145) = 5.95

BC (552)(9)

Interpretation of an Odds Ratio

(OR)

• OR = 1 implies no association.

• Assuming statistical significance:

– OR = 2 suggests cases were twice as

likely as controls to be exposed.

– OR<1 suggests a protective factor.

Odds Ratio (cont’d)

• An OR provides a good

approximation of risk when:

– Controls are representative of a target

population.

– Cases are representative of all cases.

– The frequency of disease in the

population is small.

Examples of Case-Control

Studies

• Young women’s cancers resulting from utero

exposure to diethylstilbestrol

• Green tea consumption and lung cancer

• Maternal anesthesia and development of fetal

birth defects

• Passive smoking at home and risk of acute

myocardial infarction

• Household antibiotic use and antibiotic resistant

pneumococcal infection

Advantages of Case-Control

Studies

• Tend to use smaller sample sizes than

surveys or prospective studies

• Quick and easy to complete

• Cost effective

• Useful for studies of rare diseases

Limitations of Case-Control

Studies

• Unclear temporal relationships between

exposures and diseases

• Use of indirect estimate of risk

• Representativeness of cases and controls

often unknown

Key Points to Remember

• Descriptive studies: cross-sectional

surveys (hypothesis generation)

• Analytic studies: ecologic, case-control,

and cohort (hypothesis testing)

Conclusion

• Study designs differ in a number of key

respects, including the unit of observation;

the unit of analysis; the timing of exposure

data in relation to occurrence of disease

endpoint; complexity; rigor; and amount of

resources required.

Study Designs: Cohort

Studies

Chapter 7

Learning Objectives

• Differentiate cohort studies from other

study

designs

• List main characteristics, advantages, and
disadvantages of cohort studies

• Describe three research questions that
lend themselves to cohort studies

• Calculate and interpret a relative risk

• Give three examples of published studies
discussed in this chapter

Temporality

• Temporality refers to the timing of

information about cause and effect.

• Did the information about cause and effect

refer to the same point in

time

?

• Or, was the information about the cause

garnered before or after the information

about the effect?

Limitations of Other Study

Designs

• Demonstrating temporality is a

difficulty of most observational

studies.

Limitations of Other Study

Designs

(cont’d)

• Cross-sectional and case-control study
designs are based on exposure and
disease information that is collected at the
same time.

• Advantage: Efficient for generating and
testing hypotheses.

• Disadvantage: Leads to challenges
regarding interpretation of results.

Limitations of Other Study
Designs (cont’d)

• Cross-sectional studies:

– Present difficulties in distinguishing the

exposures from the outcomes of the disease,

especially if the outcome marker is a

biological or physiological parameter.

Limitations of Other Study
Designs (cont’d)

• Case-control studies:

– Raise concerns that recall of past

exposures differs between cases and

controls.

Limitations of Other Study
Designs (cont’d)

• There has been no actual lapse of time

between measurement of exposure and

disease.

• None of the previous study designs is well

suited for uncommon exposures.

What is a cohort?

• A cohort is defined as a population group,

or subset thereof, that is followed over a

period of time.

• The term cohort is said to originate from

the Latin cohors, which referred to one of

ten divisions of an ancient Roman legion.

What is a cohort? (cont’d)

• Cohort group members experience a

common exposure associated with a

specific setting (e.g., an occupational

cohort or a school cohort) or they share a

non-specific exposure associated with a

general classification (e.g., a birth

cohort—being born in the same year or

era).

Cohort Effect

• The influence of membership in a particular
cohort.

• Example: Tobacco use in the U.S.

– Fewer than 5% of population smoked around the
early 1900s.

– Free cigarettes for WWI troops increased
prevalence of smoking in the population.

– During WWI, age of onset varied greatly; then
people began smoking earlier in life.

– One net effect was a shift in the distribution of the
age of onset of lung cancer.

Cohort Analysis

• The tabulation and analysis of morbidity or

mortality rates in relationship to the ages

of a specific group of people (cohort)

identified at a particular period of time and

followed as they pass through different

ages during part or all of their life span.

Wade Hampton Frost

• Popularized cohort analysis method.

• Arranged tuberculosis mortality rates in a

table with age on one axis and year of

death on the other.

• One can quickly see the age-specific

mortality for each of the available years on

one axis and the time trend for each age

group on the other.

Wade Hampton Frost

Life Table Methods

• Give estimates for survival during time

intervals and present the cumulative

survival probability at the end of the

interval.

• Example: Life tables can be constructed to

portray the survival times of patients in

clinical trials.

Life Table Methods (cont’d)

• There are two life table methods:

– Cohort Life Table

– Period (Current) Life Table

Life Table Methods (cont’d)

• Cohort life table:

– Shows the mortality experience of all persons

born during a particular year, such as 1900.

• Period life table:

– Enables us to project the future life

expectancy of persons born during the year

as well as the remaining life expectancy of

persons who have attained a certain age.

Describing the Mortality

Experience of the Population

• Years of Potential Life Lost (YPLL)

• Disability-adjusted life years (DALYs)

YPLL

• Years of potential life lost (YPLL)

– Computed for each individual in a

population by subtracting that person’s
life span from the average life

expectancy of the population

DALYs

• Disability-adjusted life years (DALYs)

– Adds the time a person has a disability

to the time lost to early death

Survival Curves

• A method for portraying survival times

• In order to construct a survival curve, the

following information is required:

– Time of entry into the

study

– Time of death or other outcome

– Status of patient at time of outcome, e.g.,

dead or censored (patient is lost to follow-up)

Cohort Studies

• Start with a group of subjects who lack a

positive history of the outcome of interest

and are at risk for the outcome

• Include at least two observation points:

one to determine exposure status and

eligibility and a second (or more) to

determine the number of incident cases

Cohort

Studies (cont’d)

• Permit the calculation of incidence rates

• Can be thought of as going from cause to effect

• The individual forms the unit of observation and

the unit of analysis.

• Involve the collection of primary data, although

secondary data sources are used sometimes

for both exposure and disease assessment

Cohort Studies

Timing of Data Collection

Sampling and Cohort Formation

Options

• Cohort studies differ according to

sampling strategy used.

• The two strategies are population-

based samples and exposure-based

samples.

Population-Based Cohort Studies

• The cohort includes either an entire
population or a representative sample of
the population.

• Population-based cohorts have been used
in studies of coronary heart disease.

Framingham Study

• Conducted in Framingham,
Massachusetts

• Ongoing study of CHD initiated in 1948

• Used a random sample of 6,500 from
targeted age range of 30 to 59 years

Tecumseh Study

• Conducted in Tecumseh, Michigan

• A total community cohort study

• Examined the contribution of
environmental and constitutional factors to
the maintenance of health and origins of
illness

• Started in 1959-1960 and enrolled 8,641
(88% of the community)

Population-Based Cohort

Studies (cont’d)

• Exposures unknown until the first period of

observation when exposure information is

collected

• Examples: After administration of

questionnaires, collection of biologic

samples, and clinical examinations, there

can be two or more levels of exposure.

Exposure-Based Cohort Studies

• These studies overcome limitations of

population-based cohort studies, which

are not efficient for rare exposures.

• Certain groups, such as occupational

groups, may have higher exposures than

the general population to specific hazards.

Definition of Exposure-Based

Cohort

• An exposure-based cohort is made up of

subjects with a common exposure.

• Examples:

– Workers exposed to lead during battery

production

– Childhood cancer survivors

– Veterans

– College Graduates

Comparison (Non-Exposed Group)

• Cohort studies involve the comparison of

disease rates between exposed and non-

exposed groups.

• The comparison group is similar in

demographics and geography to the

exposed group, but lacks the exposure.

• In an occupational setting, several

categories of exposure may exist.

Outcome Measures

• Discrete Events

–Single events and multiple occurrences

• Levels of Disease Markers

• Changes in Disease Markers

–Rate of change, change in level within

time

Temporal Differences in Cohort

Designs

• There are several variations in cohort

designs that depend on the timing of data

collection.

• These variations are:

– prospective cohort studies

– retrospective cohort studies

Prospective Cohort Study

• Purely prospective in nature;

characterized by determination of

exposure levels at baseline (the present),

and follow-up for occurrence of disease at

some time in the future

Advantages of Prospective

Cohort Studies

• Enable the investigator to collect data on

exposures; the most direct and specific

test of the study hypothesis

• The size of the cohort is under greater

control by the investigators

Advantages of Prospective Cohort

Studies (cont’d)

• Biological and physiological assays can be

performed with decreased concern that the

outcome will be affected by the underlying

disease process.

• Direct measures of the environment (e.g.,

indoor radon levels, electromagnetic field

radiation, cigarette smoke concentration) can

be made.

Retrospective Cohort Study

• Despite substantial benefits of prospective

cohort studies, investigators have to wait

for cases to accrue.

• Retrospective cohort studies make use of

historical data to determine exposure level

at some baseline in the past.

Advantages of Retrospective

Cohort Studies

• A significant amount of follow-up may
be accrued in a relatively short period
of time.

• The amount of exposure data
collected can be quite extensive and
available to the investigator at minimal
cost.

Historical Prospective Cohort Study

• A design that makes use of both

retrospective features (to determine

baseline exposure) and prospective

features (to determine disease incidence

in the future)

• Also known as an ambispective cohort

study

Practical Considerations Regarding

Cohort Studies

• Availability of exposure data

• Size and cost of the cohort used

• Data collection and data management

• Follow-up issues

• Sufficiency of scientific justification

Availability of Exposure Data

• High quality historical exposure data are

absolutely essential for retrospective

cohort studies.

• Need to trade off between a retrospective

study design (with the benefits of more

immediate follow-up time) and collection of

primary exposure data in a prospective

cohort design.

Size and Cost of the Cohort

• The larger the size of the cohort, the

greater the opportunity to obtain findings

in a timely manner.

• Resource constraints typically influence

design decisions.

Data Collection and Data

Management

• Larger studies are more demanding than

smaller ones; challenges due to data

collection and data management.

• Explicit protocols for quality control (e.g.,

double entry of data and scannable

forms) should be considered in the

design and implementation stage.

Data Collection and Data

Management (cont’d)

• Organizational and administrative burdens are

increased when there are multiple levels of data

collection (such as phone interviews, mailed

questionnaires, consent forms to access

medical records).

Follow-up Issues

• There are two types of follow-up:

–Active follow-up

–Passive follow-up

Active Follow-up

• The investigator, through direct contact

with the cohort, must obtain data on

subsequent incidence of the outcome

(disease, change in risk factor, change in

biological marker).

• Accomplished through follow-up mailings,

phone calls, or written invitations to return

to study sites/centers.

Active Follow-up (cont’d)

• Example: Minnesota Breast Cancer

Family Study

– Mailed survey

– A reminder postcard 30 days later

– A second survey

– A telephone call to non-responders

Passive Follow-up

• Does not require direct contact with cohort

members.

• Possible when databases containing the

outcomes of interest are collected and

maintained by organizations outside the

investigative team.

• Example: Used in the Iowa Women’s
Health Study.

Sufficiency of Scientific

Justification

• There should be considerable scientific
rationale for a cohort study.

• Additional justification for cohort studies may
come from laboratory experiments or animal
studies.

• Cohort studies are the only observational study
design that permits examination of multiple
outcomes.

Cohort Studies:

Measures of Effect

• Relative risk is the ratio of the risk of disease or

death among the exposed to the risk among the

unexposed.

• Recall that risk is estimated in epidemiologic

studies only by the cumulative incidence.

• When the relative risk is calculated with

incidence rates or incidence density, then the

term rate ratio is more precise.

Relative Risk

Relative risk =

Incidence rate in the exposed

Incidence rate in the non-exposed

Relative Risk

• Using the notation from the 2 by 2

table, the relative risk can be

expressed as

[A/(A+B)] / [C/(C+D)]

Measures of Association

(cont’d)

Disease

Status

Incidence

Exposure Yes No Totals Total

Status

Yes A B A+B A/(A+B)

No C D C+D C/(C+D)

A + C B + D N

Relative Risk [A/A+B]/[C/C+D]

Cohort Studies:

Sample Calculation

• Is there an association between child abuse

and suicide attempts among chemically

dependent adolescents?

• Source: Deykin EY, Buka SL. Am J Public

Health. 1994;84:634-639.

Sample Calculation (cont’d)

Examples of Major Cohort Studies

• The Alameda County Study

– Studied factors associated with health and
mortality

– Involved residents of Alameda County, CA,
ages 16-94 years

– Data collected through mailed
questionnaires; telephone interviews or home
interviews of non-respondents

– Follow-up with same procedures at years 9,
18, and 29

Examples of Major Cohort Studies

(cont’d)

• Honolulu Heart Program

– Studied coronary heart disease and stroke in

men of Japanese ancestry

– Involved men of Japanese ancestry living on

Oahu, HI, ages 45-65 years

– Data were collected through mailed

questionnaires, interviews, and clinic

examinations.

Examples of Major Cohort

Studies (cont’d)

• Nurses’ Health Study

– Originally studied oral contraceptive use;

expanded to women’s health

– Married female R.N.s ages 30-55 years

– Data collected through mailed questionnaires

– Follow-up every 2 years; toenail sample at

year 6 and blood sample at year 13

Nested Case-

Control Studies

• A nested case-control study is defined as a type

of case-control study “. . . in which cases and

controls are drawn from the population in a

cohort study.”

• Example: nested case-control breast cancer

study

– Controls are a subset of the source population for

the cohort study of breast cancer.

– Cases of breast cancer identified from the cohort

study would comprise the cases.

Advantages of Nested Case-

Control Studies

• Provide a degree of control over

confounding factors.

• Reduce cost because exposure

information is collected from a subset of

the cohort only.

• An example is an investigation of suicide

among electric utility workers.

Strengths of Cohort Studies

• Permit direct determination of risk.

• Time sequencing of exposure and

outcome.

• Can study multiple outcomes.

• Can study rare exposures.

Limitations of Cohort Studies

• Take a long time

• Costly

• Subjects lost to follow-up

Table 7-6

• Table 7-6 summarizes various study

designs by comparing their characteristics,

advantages, and disadvantages.