Cohort Studies Flashcards
What is a cohort study ?
Any study in which groups of people with defined characteristics are followed up to determine the incidence of, or mortality from, some specific disease, cause of death, or some other outcome
A longitudinal, observational study design that follows a group (cohort) of individuals over time to examine the relationship between exposures and outcomes
Participants are classified based on their exposure status before the outcome occurs
Cohort studies are prospective (forward-looking) or retrospective (historical data used for follow-up)
Describe the strengths of a cohort study:
Establishes temporal relationships – confirms exposure precedes the outcome, strengthening causal inference
Measures multiple outcomes – one exposure can be linked to multiple diseases
Minimises recall bias - especially in prospective studies where data is collected before disease onset
Can calculate absolute risk - Allows for direct measurement of incidence rate and risk ratios (relative risk, RR)
Efficient for studying rare exposures - suitable for occupational/environmental exposures
Describe the limitations of a cohort study:
Time consuming and expensive - long follow up periods increase costs and logistical challenges
Loss to follow up - attrition bias, participants may drop out which affects study validity
Limited use for studying rare diseases - as if the outcome is uncommon, large sample sizes are needed
Confounding bias - other factors may influence the exposure-outcome relationship, requiring statistical adjustments
Changes in exposure over time - behaviour/lifestyle factors may change, complicating analysis
Generalisability
Describe the key features of a cohort study:
Temporal sequence - exposure is measured before the outcome develops
Involves at least two groups – exposed vs. non-exposed
Follow up over time - observes participants for disease development or other health outcomes
Multiple outcomes - can assess several outcomes from a single exposure
Less prone to recall bias than case-control studies (since exposure is measured prospectively
What’s the difference between aetiologic and non-aetiologic studies?
Aetiologic studies:
-Aim to identify the causes (aetiology) of disease
- They examine whether an exposure increases or decreases the risk of a specific outcome
- lifestyle e.g diet, physical activity, smoking
- physical and physiological characteristics
- Socioeconomic/demographic parameters
- environmental e.g pollution, radiation, occupational exposure
Non-aetiologic:
- Do not focus on cause-effect relationships.
- Instead, they might describe disease distribution, burden, or help in surveillance, diagnosis, or health service planning
- Health care interventions/treatment (non-random comparisons)
- drug exposures
- biomarkers of disease progression
What outcomes can be examined in cohort studies ?
Health related outcomes:
- occurrence of disease/ death
- traits e.g, smoking, obesity
- disability
- disease progression
- premalignant to malignant progression
- cognitive decline
Non-health related outcomes:
- educational attainment
- employment
- social isolation
How may outcomes be measured ?
Active follow up:
- contacting recruits
- repeated measures of outcomes
- advantage is better outcome definition
- disadvantages include expensive, time consuming and attrition
Passive follow up;
- record linkage
- morbidity
- advantages include less expensive, non-invasive and less attrition
- disadvantages are poorer outcome definition
Which groups may be studied in cohort studies ?
General Population Cohorts – Large-scale studies of representative populations
Occupational Cohorts – Workers exposed to chemicals, radiation, or pollutants
Birth & Pediatric Cohorts – Follow-up from birth to adulthood
Disease-Specific Cohorts – Patients with pre-existing conditions monitored for disease progression
High-Risk Populations – Military personnel, healthcare workers, smokers, genetic predisposition groups
Describe the statistical tests used during a cohort study:
Incidence rate = (new cases during follow-up) / (total person-time at risk).
Relative Risk (RR) = Incidence in exposed / Incidence in non-exposed.
Hazard ratios (HR) used in survival analysis (e.g., Cox proportional hazards model).
What types of exposures can be measured in cohort studies ?
Cohort studies can examine a wide variety of exposures, including biological, behavioral, environmental, occupational, and socioeconomic factors.
These may be voluntary (e.g. smoking, diet, exercise), involuntary (e.g. air pollution, passive smoking), or categorical (e.g. sex, genetic predisposition).
Exposures must be measurable at baseline and ideally precede the outcome of interest to infer temporal associations
