Module 9 Flashcards
Cohort
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)
Cohort studies
o Cohorts followed over time
o See who develops disease
o Incorporates passage of time
Advantages of cohort studies
o Cause precedes effect
o Low recall bias
o Multiple outcomes can be studied
Types of cohorts
- Open/dynamic
- Fixed
- Closed
Open/dynamic cohort
o Defined by changeable characteristic, e.g., smoking
o Members come and go with losses to follow-up
Fixed cohort
o Defined by unchangeable event—exposure to atom bomb, giving birth
o No new members, but losses may occur
Closed cohort
o Defined by unchangeable event
o No new members, no losses
o Short term, e.g., food poisoning at church picnic
Assumption of cohort studies
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)
Internal comparison (cohort studies)
Unexposed members of same cohort
General population comparison (cohort studies)
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
Comparison cohort
Unexposed cohort from another population
Temporal differences in cohort designs
Variations in cohort designs depend on timing of data collection
- Prospective cohort studies (longitudinal)
- Retrospective cohort studies
- Historical prospective cohort studies
Measures of effect
- Relative risk
2. Attributable risk
Relative Risk
RR= incidence rate in exposed/incidence rate in unexposed
- RR 1.0: malicious effect
- RR = 1.0: no effect
Attributable Risk (rate difference)
Incidence rate in the exposed-Incidence rate in unexposed
*Measures potential savings if risk factor eliminated from population
Dose-response relationships
Calculate RR for each dose
Dose response strengthens inferences
Nested case-control studies
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
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.
Ex: an investigation of suicide among electric utility workers
Challenges of cohort studies
o Maintaining follow-up
o Bias
o Confounding
Loss to follow-up (cohort)
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
Selection bias in cohort study
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
Confounding factor
o Factor relates to disease or risk factor
o Stratify data by confounding factor
o Multivariate analysis
Strategies to deal with confounding
o Restrict study population
o Control confounding in the data analysis
o Use stratified analysis or multivariate analysis
Advantages of cohort studies
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
Disadvantages of cohort studies
o Expensive o Time consuming o Long follow-up o Changes in exposure o Attrition (lost to follow-up)
Environmental epidemiology
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
Environmental hazards
- Chemical agents
- Electromagnetic radiation
- Ionizing radiation
- Heavy metals
- Air pollution
- Temperature increases from global warming/climate change
Occupational epidemiology
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
Hazardous agents in the work environment
• Biological Exposure – Molds, bacteria, viruses, etc • Chemical Exposure – Hazardous waste, exhaust/fumes, heavy metals, etc • Physical – Radiation, heat/cold, noise, etc
Health effects associated w/ work environment
- Risks for pregnant workers/unborn fetus
- Lung diseases
- Dermatologic problems
- Certain cancers among occupational groups
- Injuries/trauma
- Psychological conditions (absenteeism, stress)
