Study Designs in Epidemiologic Research Thomas Songer, Phd modified by Supercourse team



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  • Thomas Songer, PhD
  • Modified by Supercourse team
  • Basic Epidemiology

Fundamental Assumption in Epidemiology

  • Disease doesn’t occur in a vacuum
    • Disease is not randomly distributed throughout a population
    • Epidemiology uses systematic approach to study the differences in disease distribution in subgroups
    • Allows for study of causal and preventive factors

Components of Epidemiology

  • Measure disease frequency
    • Quantify disease
  • Assess distribution of disease
    • Who is getting disease?
    • Where is disease occurring?
    • When is disease occurring?
    • Formulation of hypotheses concerning causal and preventive factors
  • Identify determinants of disease
    • Hypotheses are tested using epidemiologic studies

Types of primary studies

  • Descriptive studies
    • describe occurrence of outcome
  • Analytic studies

Basic Question in Analytic Epidemiology

  • Are exposure and disease linked?
  • E
  • D
  • Exposure
  • Disease

Basic Questions in Analytic Epidemiology

  • Look to link exposure and disease
    • What is the exposure?
    • Who are the exposed?
    • What are the potential health effects?
    • What approach will you take to study the relationship between exposure and effect?
  • Wijngaarden

Basic Research Study Designs and their Application to Epidemiology

Big Picture

  • To prevent and control disease
  • In a coordinated plan, look to
    • identify hypotheses on what is related to disease and may be causing it
    • formally test these hypotheses
    • Study designs direct how the investigation is conducted

What designs exist to identify and investigate factors in disease?

  • Study Designs
  • Case report
  • Case series
  • Descriptive
  • Epidemiology
  • Descriptive
  • RCT
  • Before-After
  • study
  • Cross-sectional
  • study
  • Case-Crossover
  • study
  • Case-Control
  • study
  • Analytic
  • Ecologic study

Timeframe of Studies

  • Prospective Study - looks forward, looks to the future, examines future events, follows a condition, concern or disease into the future
  • time

Timeframe of Studies

  • Retrospective Study - “to look back”, looks back in time to study events that have already occurred
  • time
  • Study begins here

Study Design Sequence

  • Case reports
  • Case series
  • Descriptive
  • epidemiology
  • Analytic
  • epidemiology
  • Clinical
  • trials
  • Animal
  • study
  • Lab
  • study
  • Cohort
  • Case-
  • control
  • Cross-
  • sectional
  • Hypothesis formation
  • Hypothesis testing
  • Descriptive Studies
  • Case-control Studies
  • Cohort Studies
  • Develop
  • hypothesis
  • Investigate it’s
  • relationship to
  • outcomes
  • Define it’s meaning
  • with exposures
  • Clinical trials
  • Test link
  • experimentally
  • Increasing Knowledge of
  • Disease/Exposure

Descriptive Studies

Case Reports

  • Detailed presentation of a single case or handful of cases
  • Generally report a new or unique finding
      • e.g. previous undescribed disease
      • e.g. unexpected link between diseases
      • e.g. unexpected new therapeutic effect
      • e.g. adverse events

Case Series

  • Experience of a group of patients with a similar diagnosis
  • Assesses prevalent disease
  • Cases may be identified from a single or multiple sources
  • Generally report on new/unique condition
  • May be only realistic design for rare disorders

Case Series

  • Advantages
      • Useful for hypothesis generation
      • Informative for very rare disease with few established risk factors
      • Characterizes averages for disorder
  • Disadvantages
      • Cannot study cause and effect relationships
      • Cannot assess disease frequency
  • Case Report
  • Case Series
  • Descriptive
  • Epidemiology Study
  • One case of unusual
  • findings
  • Multiple cases of
  • findings
  • Population-based
  • cases with denominator

Analytical Studies

Study Designs - Analytic Epidemiology

  • Experimental Studies
    • Randomized controlled clinical trials
    • Community trials
  • Observational Studies
    • Group data
      • Ecologic
    • Individual data
      • Cross-sectional
      • Cohort
      • Case-control
      • Case-crossover

Experimental Studies

  • treatment and exposures occur in a “controlled” environment
  • planned research designs
  • clinical trials are the most well known experimental design. Clinical trials use randomly assigned data.
  • Community trials use nonrandom data

Observational Studies

  • non-experimental
  • observational because there is no individual intervention
  • treatment and exposures occur in a “non-controlled” environment
  • individuals can be observed prospectively, retrospectively, or currently

Cross-sectional studies

  • An “observational” design that surveys exposures and disease status at a single point in time (a cross-section of the population)
  • time
  • Study only exists at this point in time

Cross-sectional Design

  • time
  • Study only exists at this point in time
  • Study
  • population
  • No Disease
  • Disease
  • factor present
  • factor absent
  • factor present
  • factor absent

Cross-sectional Studies

  • Often used to study conditions that are relatively frequent with long duration of expression (nonfatal, chronic conditions)
  • It measures prevalence, not incidence of disease
  • Example: community surveys
  • Not suitable for studying rare or highly fatal diseases or a disease with short duration of expression

Cross-sectional studies

  • Disadvantages
      • Weakest observational design, (it measures prevalence, not incidence of disease). Prevalent cases are survivors
      • The temporal sequence of exposure and effect may be difficult or impossible to determine
      • Usually don’t know when disease occurred
      • Rare events a problem. Quickly emerging diseases a problem

Epidemiologic Study Designs

  • Case-Control Studies
    • an “observational” design comparing exposures in disease cases vs. healthy controls from same population
    • exposure data collected retrospectively
    • most feasible design where disease outcomes are rare

Case-Control Studies

  • Cases: Disease
  • Controls: No disease

Case-Control Design

  • Study
  • population
  • Cases
  • (disease)
  • Controls
  • (no disease)
  • factor present
  • factor absent
  • factor present
  • factor absent
  • present
  • past
  • time
  • Study begins here

Case-Control Study

  • Strengths
    • Less expensive and time consuming
    • Efficient for studying rare diseases
  • Limitations
    • Inappropriate when disease outcome for a specific exposure is not known at start of study
    • Exposure measurements taken after disease occurrence
    • Disease status can influence selection of subjects

Hypothesis Testing: Case-Crossover Studies

  • Study of “triggers” within an individual
  • ”Case" and "control" component, but information of both components will come from the same individual
  • ”Case component" = hazard period which is the time period right before the disease or event onset
  • ”Control component" = control period which is a specified time interval other than the hazard period

Epidemiologic Study Designs

  • Cohort Studies
    • an “observational” design comparing individuals with a known risk factor or exposure with others without the risk factor or exposure
    • looking for a difference in the risk (incidence) of a disease over time
    • best observational design
    • data usually collected prospectively (some retrospective)

Cohort Design

  • time
  • Study begins here
  • Study
  • population
  • free of
  • disease
  • Factor
  • present
  • Factor
  • absent
  • disease
  • no disease
  • disease
  • no disease
  • present
  • future

Timeframe of Studies

  • Prospective Study - looks forward, looks to the future, examines future events, follows a condition, concern or disease into the future
  • time
  • Study begins here

Prospective Cohort study

  • Measure exposure
  • and confounder
  • variables
  • Exposed
  • Non-exposed
  • Outcome
  • Outcome
  • Baseline
  • time
  • Study begins here

Timeframe of Studies

  • Retrospective Study - “to look back”, looks back in time to study events that have already occurred
  • time
  • Study begins here

Retrospective Cohort study

  • Measure exposure
  • and confounder
  • variables
  • Exposed
  • Non-exposed
  • Outcome
  • Outcome
  • Baseline
  • time
  • Study begins here

Cohort Study

  • Strengths
    • Exposure status determined before disease detection
    • Subjects selected before disease detection
    • Can study several outcomes for each exposure
  • Limitations
    • Expensive and time-consuming
    • Inefficient for rare diseases or diseases with long latency
    • Loss to follow-up

Experimental Studies

  • investigator can “control” the exposure
  • akin to laboratory experiments except living populations are the subjects
  • generally involves random assignment to groups
  • clinical trials are the most well known experimental design
  • the ultimate step in testing causal hypotheses

Experimental Studies

  • In an experiment, we are interested in the consequences of some treatment on some outcome.
  • The subjects in the study who actually receive the treatment of interest are called the treatment group.
  • The subjects in the study who receive no treatment or a different treatment are called the comparison group.

Epidemiologic Study Designs

  • Randomized Controlled Trials (RCTs)
    • a design with subjects randomly assigned to “treatment” and “comparison” groups
    • provides most convincing evidence of relationship between exposure and effect
    • not possible to use RCTs to test effects of exposures that are expected to be harmful, for ethical reasons

Experimental Design

  • time
  • Study begins here (baseline point)
  • Study
  • population
  • Intervention
  • Control
  • outcome
  • no outcome
  • outcome
  • no outcome
  • baseline
  • future
  • RANDOMIZATION

Epidemiologic Study Designs

  • Randomized Controlled Trials (RCTs)
    • the “gold standard” of research designs
    • provides most convincing evidence of relationship between exposure and effect
      • trials of hormone replacement therapy in menopausal women found no protection for heart disease, contradicting findings of prior observational studies

Randomized Controlled Trials

  • Disadvantages
    • Very expensive
    • Not appropriate to answer certain types of questions
      • it may be unethical, for example, to assign persons to certain treatment or comparison groups

Review Questions (Developed by the Supercourse team)

  • Describe the link between exposure and disease
  • Describe study design sequence
  • Describe strengths and weaknesses of each design


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