HK 207_Chapter 7
Introduction to Epidemiology
Analytic Epidemiology
Focuses on finding and interpreting causal relationships between exposures and health outcomes.
Involves comparison groups to understand relationships,
A key component in planning public health interventions.
Learning Objectives
Study Design Differences: Identify three distinctions across study types
Study Description: Explain case-control, ecological, and cohort studies
Calculations: Compute odds ratios, relative risk, and attributable risk
Trial Types: Recognize uses of randomized controlled trials and quasi-experimental designs
Bias Awareness: Define sources of bias in epidemiologic studies
Types of Epidemiologic Studies
Descriptive Studies
Case report: Detailed report of a single case.
Case series: Collection of case reports involving patients with similar diagnoses.
Cross-sectional studies: Snapshot of health outcomes and exposures at one point in time.
Analytic Studies
Case-control: Compare individuals with disease (cases) to those without (controls).
Cohort: Follows a group over time to observe outcomes based on exposure.
Ecological: Studies population-level data rather than individual data.
Experimental (intervention) studies: Randomized trials testing interventions.
Importance of Analytic Studies
Disease Prevention: Helps to prevent diseases by identifying risk factors.
Quantitative Evaluations: Key for assessing intervention programs.
Drug Efficacy and Safety: Essential in evaluating new medications and treatment procedures.
Categories of Analytic Studies
Observational Studies
No control over exposure factors; naturally occurring groups.
Conditions for subjects are not assigned randomly.
Experimental Studies
Control over exposure factors; subjects are randomly assigned to conditions.
Design Considerations
Control of Exposure: Who manipulates the exposure factor?
Observations: Number of observations involved.
Directionality: Timing of exposure measurement.
Data Collection Methods: Techniques used for data gathering.
Timing: When data is collected in relation to exposures and outcomes.
Unit of Observation: Individual or groups being studied.
Subject Availability: Accessibility of participants in the study.
Observational Analytic Studies
Types
Ecologic Studies: Focus on populations rather than individuals.
Case-control Studies: Cases with a disease compared against controls without the disease.
Cohort Studies: Longitudinal studies tracking groups over time.
Ecologic Studies
Definition: Analyze groups (e.g., populations) using aggregate data.
Types:
Ecologic Comparison Study: Examines exposure rates and disease rates for groups at same time.
Ecologic Correlation: Association between variables measured at the group level.
Ecologic (Ecological) Fallacy:
Incorrect inference from association at group level to individual level.
Example: Higher CHD in wealthier cities erroneously inferred to higher rates amongst wealthier individuals.
Advantages and Disadvantages of Ecologic Studies
Advantages:
Contextual health information.
Utilize group data for faster assessments.
Minimal resources needed.
Disadvantages:
Potential for ecologic fallacy.
Imprecise exposure measurements.
Case-Control Studies
Definition: Identifies subjects based on existing outcome.
Structure: Cases vs. controls, may include matched designs based on criteria like age or sex to control confounding variables.
Odds Ratio (OR)
A measure of association in case-control studies that describes the odds of exposure amongst cases relative to controls.
Calculation: OR = (AD)/(BC)
Calculation Example
Using a 2x2 table:
Case status (Yes/No) based on exposure status (A/C for cases, B/D for controls).
Odds Ratio calculated as (9x88)/(4x95) = 2.08
Interpretation
Odds Ratio > 1: Positive association between exposure and disease.
OR of 2: Odds of disease are twice as high among exposed.
OR < 1: Exposure may be protective.
OR = 1: No association.
Advantages and Disadvantages of Case-Control Studies
Advantages:
Useful for studying rare outcomes.
Quick, economical, requiring fewer subjects.
Disadvantages:
Possible inaccuracies in exposure measurement.
Difficulty in establishing temporal relationships.
Cohort Studies
Definition: Follows a group sharing a characteristic over time.
Examples: Pregnancy cohorts, occupational cohorts, school cohorts.
Types of Cohort Studies
Prospective: Participants followed forward in time after exposure classification.
Retrospective: Using historical data to assess prior exposure levels.
Relative Risk (RR)
A measure used in cohort studies to compare incidence of outcomes between exposed and nonexposed groups.
Formula: RR = Incidence rate in exposed / Incidence rate in unexposed.
Calculation Example
Relative Risk calculation using the 2x2 table: RR = (3/107)/(2/603) = 8.43
Advantages and Disadvantages of Cohort Studies
Advantages:
Directly measure risk, temporal relationships established.
Disadvantages:
Expensive and time-consuming, loss to follow-up possible.
Experimental Studies
Intentional changes in the subject's status to test hypotheses.
Types:
Randomized Controlled Trial (RCT): Random assignment of participants to study groups.
Quasi-experimental: Investigator manipulates factors without random assignment.
Validity of Study Designs
Types
External Validity: Generalizability of study results.
Internal Validity: Dependence on sound methodology.
Bias: Systematic errors affecting study results.
Types of Bias in Epidemiologic Studies
Hawthorne Effect: Changes in behavior due to awareness of being studied.
Recall Bias: Cases remembering exposures better than controls.
Selection Bias: Distorts results from selection or participation factors.
Confounding: Distortion of exposure/outcome associations due to related factors.
Conclusion
Analytic study designs include:
Ecological studies
Case-control studies
Cohort studies
Experimental designs (RCTs and quasi-experimental)
Awareness of biases is crucial in interpreting results.