Module 9

Question 1

Cohort

Answer 1

Any group of people followed over time
Cohort group members experience a common exposure associated with:
– A specific setting (ex: occupational or school)
– A non-specific exposure associated with a general classification (ex: birth cohort)

Question 2

Cohort studies

Answer 2

o Cohorts followed over time
o See who develops disease
o Incorporates passage of time

Question 3

Advantages of cohort studies

Answer 3

o Cause precedes effect
o Low recall bias
o Multiple outcomes can be studied

Question 4

Types of cohorts

Answer 4

1. Open/dynamic
2. Fixed
3. Closed

Question 5

Open/dynamic cohort

Answer 5

o Defined by changeable characteristic, e.g., smoking

o Members come and go with losses to follow-up

Question 6

Fixed cohort

Answer 6

o Defined by unchangeable event—exposure to atom bomb, giving birth
o No new members, but losses may occur

Question 7

Closed cohort

Answer 7

o Defined by unchangeable event
o No new members, no losses
o Short term, e.g., food poisoning at church picnic

Question 8

Assumption of cohort studies

Answer 8

Exposed/unexposed group same
How to achieve comparability:
o By design—choose similar subjects (match by age, gender, race, etc.
o By stratification (in analysis)

Question 9

Internal comparison (cohort studies)

Answer 9

Unexposed members of same cohort

Question 10

General population comparison (cohort studies)

Answer 10

o Use available data on disease occurrence and death in general population
o Common in occupational studies
o Problem—healthy worker effect: bias due to fact that rate of disease/death in working population is lower than general population

Question 11

Comparison cohort

Answer 11

Unexposed cohort from another population

Question 12

Temporal differences in cohort designs

Answer 12

Variations in cohort designs depend on timing of data collection

1. Prospective cohort studies (longitudinal)
2. Retrospective cohort studies
3. Historical prospective cohort studies

Question 13

Measures of effect

Answer 13

1. Relative risk

2. Attributable risk

Question 14

Relative Risk

Answer 14

RR= incidence rate in exposed/incidence rate in unexposed

• RR 1.0: malicious effect
• RR = 1.0: no effect

Question 15

Attributable Risk (rate difference)

Answer 15

Incidence rate in the exposed-Incidence rate in unexposed

*Measures potential savings if risk factor eliminated from population

Question 16

Dose-response relationships

Answer 16

Calculate RR for each dose

Dose response strengthens inferences

Question 17

Nested case-control studies

Answer 17

A type of case-control study in which cases and controls are drawn from the population in a cohort study

• The population of the cohort study comprises both exposed and non-exposed persons
• Compare exposures of diseased (cases) to sample of controls from a cohort of subjects being studied

Question 18

Advantages of nested case-control studies

Answer 18

• Provide a degree of control over confounding factors.
• Reduce cost because exposure information is collected from a subset of the cohort only.
  Ex: an investigation of suicide among electric utility workers

Question 19

Challenges of cohort studies

Answer 19

o Maintaining follow-up
o Bias
o Confounding

Question 20

Loss to follow-up (cohort)

Answer 20

o Subjects decide to “drop out”
o Investigators lose track
Bad: Reduces sample size; those who drop out different from those who stay in
*Baseline info helps to locate subjects

Question 21

Selection bias in cohort study

Answer 21

If choice of exposed/unexposed related to outcome: those who volunteer into exposed group more likely to have outcome
*Retrospective cohort more prone because outcome already occurred

Question 22

Confounding factor

Answer 22

o Factor relates to disease or risk factor
o Stratify data by confounding factor
o Multivariate analysis

Question 23

Strategies to deal with confounding

Answer 23

o Restrict study population
o Control confounding in the data analysis
o Use stratified analysis or multivariate analysis

Question 24

Advantages of cohort studies

Answer 24

o Determines incidences directly
o Directly measures RR; direct determination of risk
o Avoids recall biases
o Time sequencing of exposure and outcome
o Can study multiple outcomes/rare exposures

Question 25

Disadvantages of cohort studies

Answer 25

o Expensive
o Time consuming
o Long follow-up
o Changes in exposure
o Attrition (lost to follow-up)

Question 26

Environmental epidemiology

Answer 26

Study of the effect on human health of physical, biologic, and chemical factors in the external environment

• Seeks to clarify the relation between physical, biologic, and chemical factors and human health.
• Aligned with toxicology and molecular and genetic epidemiology

Question 27

Environmental hazards

Answer 27

• Chemical agents
• Electromagnetic radiation
• Ionizing radiation
• Heavy metals
• Air pollution
• Temperature increases from global warming/climate change

Question 28

Occupational epidemiology

Answer 28

Application of epidemiologic methods to study populations of workers
Involves evaluation of workers:
– Exposed to chem/bio/phys agents to determine if exposures result in risk of adverse health outcomes
– With a common adverse health outcome to determine if an agent or set of agents may explain disease

Question 29

Hazardous agents in the work environment

Answer 29

• Biological Exposure
– Molds, bacteria, viruses, etc
• Chemical Exposure
– Hazardous waste, exhaust/fumes, heavy metals, etc
• Physical
– Radiation, heat/cold, noise, etc

Question 30

Health effects associated w/ work environment

Answer 30

• Risks for pregnant workers/unborn fetus
• Lung diseases
• Dermatologic problems
• Certain cancers among occupational groups
• Injuries/trauma
• Psychological conditions (absenteeism, stress)