Epidemiological Study Designs Overview

Epidemiological Study Designs

• Epidemiological studies can be descriptive or analytical
• Descriptive studies analyze the distribution of diseases or health problems in a community
• Analytical studies analyze the factors that either support or hinder disease occurrence in populations

Types of Epidemiological Study Designs

• Observational Studies:

  • Descriptive Studies:

    • Case report study: a detailed report of patient symptoms, diagnosis, treatment, and follow-up. May focus on unusual or rare occurrences.
    • Case series study: tracks subjects with similar exposure (like treatment) and evaluates outcomes.
    • Correlation study: identifies relationships between variables.
    • Cross-sectional study: observes the distribution of disease and risk factors at a specific time.

  • Analytical Studies:

    • Case-control study: compares individuals with a disease to those without.
    • Cohort study: a group of people (cohort) free of a disease, followed over time to assess the risk of developing the disease based on exposure to a risk factor.

• Experimental (Interventional) Studies:

  • Preventive trials
  • Clinical trials
  • Population trials

A. Descriptive Studies

• Focuses on the basic triad of descriptive epidemiology: time, place, and person.
• Person factors: distribution by age, sex, race, marital status, occupation, lifestyle (diet, smoking, medication)
• Place factors: geographical distribution of diseases (urban vs. rural, variation between countries)
• Time factors: secular trends, seasonal variations. Helps to formulate hypotheses about disease determinants

a. Case Report

• Detailed report of an individual patient's symptoms, diagnosis, treatment, and follow-up
• Reports may focus on unusual or novel occurrences

b. Case Series

• Medical research study tracking subjects with known exposure (similar treatment)
• Examines medical records to assess exposure and outcome

Analytic Studies

• 1- Cross-sectional study:

  • Provides a snapshot of a current situation at a specific time
  • Primarily descriptive but may also analyze the relationship between disease and risk factors in relation to time, place, and person.
  • Measures prevalence, not incidence
  • Not suitable for studying rare or highly fatal diseases, or diseases with short durations

• 2- Case-control study:

  • Compares persons with a disease (cases) with those who do not have the disease (controls).
  • Examines their past exposures to possible risk factors of interest.
  • Advantages: small number of subjects, relatively quick results, low cost.
  • Disadvantages: selection bias, information bias, recall bias, confounding bias. Useful for exploring rare diseases

• 3- Cohort study:

  • A group of people (cohort) free of a disease, followed over time to assess the risk of developing the disease based on exposure to a risk factor.
  • Classifies subjects based on their exposure status (exposed or not exposed).
  • Advantages: lack of bias in exposure assessment, calculate incidence rates, relative/attributable risk estimation, observation of additional disease development
  • Disadvantages: require large numbers of subjects, long follow-up periods, attrition, and potential changes in diagnostic criteria or methods. Useful when exposure is rare, but incidence among exposed is high.

Types of Cohort Studies

• a. Concurrent prospective cohort study: Observational study done in the present.
• b. Non-Concurrent prospective cohort study/Historical/Retrospective cohort study: Exposure has occurred in the past.

Advantages of Cohort Studies

• A lack of bias because exposure is assessed prior to disease development.
• Calculated incidence rates (relative risk, attributable risk)
• Can observe subsequent/additional diseases

Disadvantages of Cohort Studies

• Large sample sizes required
• Lengthy follow-up periods
• Attrition (loss of subjects)
• Potential for changes in diagnostic methods

Interventional Studies

• Clinical trial
• Quasi-experimental study

Clinical Trials

• 1. Randomization: The investigator randomly assigns participants to exposed or non-exposed groups.
• 2. Blinding: May include single, double, or triple blinding (subject, data collector, and/or analysts blind to exposure status)

Problems in Clinical/Epidemiological Studies

• Ethical concerns
• Non-compliance/attrition
• Defining inclusion/exclusion criteria
• Sample size
• Associated costs
• Long follow-up periods

Stages of Clinical Trials

• Phase 1: Human safety (tens of subjects)
• Phase 2: Expanded safety (hundreds of subjects)
• Phase 3: Efficacy and safety (thousands of subjects)
• Phase 4: Long-term safety and effectiveness evaluation (several hundreds/thousands of subjects)