Randomized Controlled Trials (RCTs) –> Gold Standard - Participants are randomly allocated to control and treatment (intervention) groups.
- Participants are followed forward in time (prospectively) from exposure to outcome.
- Continuous Outcome
 - The outcome can be either an increasing or decreasing variable.
- Discrete Outcome
 - The outcome is either yes or no, not a range of options.
RCT Strengths - Random allocation: Ensures groups are similar (known and unknown factors).
- Temporality: Longitudinal follow-up ensures that exposure precedes the outcome.
- This is one of the criteria for establishing causality.
Weaknesses - Cost
- Length of time required for follow-up until the outcome is observed.
- Generalizability: Patients who agree to participate may differ from the target population to whom the study is intended to apply.
- Ethics & Feasibility: Not always ethical or logistically feasible/suitable.
| Cohort-Analytic Studies - Participants are not randomly allocated to control and treatment (intervention) groups.
- Participants are followed forward in time (prospectively) from exposure to outcome.
Strengths - Cost/Ethics & Feasibility: Potentially less costly and/or more feasible than an RCT (if it is not ethical/feasible to randomize).
- It may not be fair to deny the intervention to the control group –> Unethical RCT.
- It may not be feasible to deliver the intervention to only some members of the eligible population –> RCT not feasible.
- Temporality: Longitudinal follow-up to ensure that exposure precedes the outcome.
Weaknesses - Selection bias: Groups may differ in ways other than exposure to the intervention.
- Baseline differences: Groups may differ in characteristics that existed before the intervention began.
- Cost: Remains relatively expensive.
| Cohort - Interested in the likelihood that people will experience or develop an outcome given their exposure to a disease, condition, or situation (prognosis question; e.g., how likely are patients with ulcerative colitis to develop bowel cancer?).
- Comparator?
Strengths - Temporality: Longitudinal follow-up to ensure that exposure precedes the outcome.
- Rare or specific exposures
- Multiple outcomes
- Best evidence for evaluating risk/prognostic factors.
Weaknesses - Expense: Follow-up may require large numbers of people to see the outcome.
- Lack of a control group: Participants may systematically differ on predictors of the outcome.
- Inefficient: For studying rare outcomes.
- Time: Follow-up may take a long time for diseases with a long latency period.
- Limited exposures: Can study only single or specific groups of exposures.
- Contamination: Long follow-ups mean other factors may change that could cause the outcome (e.g., differential care and treatment, exposure to other causative agents).
| Case-Control - Participants with and without the outcome are identified.
- Investigators look back in time (retrospectively) from outcome to exposure.
- Investigators often try to match cases and controls: this helps to ensure groups are as similar as possible regarding important variables that may influence the outcome (e.g., age, sex).
Strengths - Cost & Time: Often more economical and quicker than a cohort study.
- Good for the study of rare outcomes and common exposures.
- Includes a control group (limits bias).
- Can examine multiple exposures.
Weaknesses - Uncertainty about Temporality: Difficult to confirm temporality.
- Exposure information: Difficult to obtain accurate information on the timing, duration, and dose of exposure (recall bias).
- Control group: Difficult to find a control group that is comparable (on factors other than the intervention).
- Cannot examine multiple outcomes.
| Cross-Sectional - A group of people are interviewed or asked to complete a survey to determine whether they have experienced an exposure of interest and an outcome of interest.
- Exposure and outcome are measured simultaneously (at one point in time).
Strengths - Cost & Time: Economical and quick – do not follow people over time.
- Good for the exploratory research phase: Can help to identify potential causal factors that can be studied with stronger designs.
Weaknesses - No Temporality: Outcome and exposure are measured at the same time.
- Exposure information: Difficult to obtain accurate information on the timing, duration, and dose of the exposure (recall bias).
- Control group: There may be no control group, or if there is, it may not be comparable.
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