Causality and Study Designs Flashcards
Association
- Relationship between two or more variables
- Does not automatically mean that one variable is the cause of change in another
Causation
-One event is the result of the occurrence of the other event
Determining Causality
- Answer about the relationship between a possible cause/exposure (independent) and an outcome (dependent)
- Bradford-Hill’s criteria for causality are used to help decide if a variable contributed to an outcome
- Study designs differ to the degree that they meet Bradford-Hill criteria
- Higher level study designs provide stronger evidence for causality
Temporality
-Cause must precede effect
Reproducibility/Consistency
-Same results with different populations and study designs
Strength
- Of association (or effect size)
- Bigger/stronger is better
Specificity
- Nothing else causes this
- Nothing else is caused by this
Dose-response/Biological Gradient
- Higher dose produces greater effect
- Lower dose produces lower effect
- Offending agent is discontinued and effect ceases
Biological Plausibility
-Relationship makes biological sense
Coherence
-Data doesn’t conflict with known facts
Analogy
- Drug class effects
- Structural similarities
Evidence Hierarchy for Study Designs
- Meta Analyses
- Systematic Reviews
- Randomized Controlled Trials
- Cohort studies
- Case-control studies
- Case reports/case series
- Clinical textbooks/expert opinions
- Animal Research
Interpreting Evidence Hierarchy
- Observation vs experimental conditions
- Lower position: lower level evidence (expert opinion, case report, case series, cross-sectional)
- Moderate level: evidence associated with case-control and cohort studies
- Highest levels: done by RCT, systematic reviews, and meta-analyses
Observational Studies
- Exposure not under investigator’s control; only observe
- Case reports/case series
- Cross-sectional
- Case-cohort
- Cohort
Experimental Studies
- Investigators control the intervention
- Randomized controlled trial
Case reports/Case Series
- Description of unique events in 1 patient (case report) or >= 2 patients (case series)
- Weakest forms of evidence: small patient numbers, no control group, many biases possible
- Good for: new conditions, novel treatments, adverse drug events, toxic exposures
- Should include literature review to provide context for the port and detailed information on the clinical course/follow-up of patients
Cross-Sectional
- Descriptions of observations
- Larger sample sizes than case studies/series
- Can measure associations between variables (which came first/compounded?)
- Good for measuring prevalence in a population at a SINGLE time point
- Evidence quality dependent on data source
Case-Control
- Start with patients who already have a condition/outcome and look back in time for etiologies/risk factors that could’ve causes that outcome
- Control group, contains similar people who didn’t experience that outcome
- Compare cases versus controls for differences in characteristics/exposures
Case-Control Uses
- Strength of association is measured by Odds ratio
- CANNOT prove causality
- Good for investigating possible etiologies or risk factors for uncommon conditions
Cohort
- Prospective/retrospective
- Already exposed to a suspected cause/risk factor and follow them forward to see outcome
- Large groups of individuals
- Can compare rates of an outcome for patients with and without the exposure (control)
- Relative risk reflects the strength of association
- Can demonstrate temporality
- Good for establishing risk factors for an outcome and calculating incidence
Prospective
Start at the present time and follow into future
Retrospective
Start at a past time and follow to the present
Randomized Controlled Trial (RCT)
- Experimental design
- Investigators control the intervention and follow patients prospectively to measure outcome
- Decide who can/can’t participate
- Patients are randomized into experimental and controlled
- Minimize biases/confounders by study design
- Good for establishing a good standard’s efficacy
- Can demonstrate causality
- Has strength of association, temporality, and dose-response
Evidence Hierarchy + Causality
- Ability to establish causation follows the evidence hierarchy
- Case-control shows association between suspected cause and outcome of interest
- Cohort shows association and establishes temporality
- RCT shows association, establishes temporality, AND demonstrates dose-response
