Types of trials

Different Types of Trials

Randomized controlled trials (RCTs) can be categorized based on study design, purpose, randomization method, and special design features.

1. Based on Study Design

a. Parallel-Group Randomized Trial

• Participants are randomized into two or more groups that receive different interventions.
• Example: A trial comparing Drug A vs. placebo, with each participant assigned to only one group.

b. Crossover Randomized Trial

• Participants receive multiple interventions in a sequential order, with a washout period in between.
• Example: Participants receive Drug A for a period, then switch to Drug B and vice versa.
• Useful when within-subject comparisons are needed.

c. Factorial Randomized Trial

• Tests multiple interventions simultaneously by randomizing participants into different combinations of treatments.
• Example: A 2×2 factorial trial testing Drug A vs. placebo and Exercise vs. No Exercise in four groups.
• Efficient for studying interaction effects between interventions.

d. Cluster Randomized Trial

• Entire groups (e.g., hospitals, schools, communities) are randomized instead of individuals.
• Example: Randomizing clinics to provide standard care vs. an enhanced HIV prevention program.
• Useful for population-level interventions.

e. Stepped-Wedge Randomized Trial

• All participants eventually receive the intervention, but the rollout is randomized in phases.
• Example: A new vaccine program introduced to different regions at different times.
• Useful when gradual implementation is required.

2. Based on Purpose

a. Superiority Trial

• Tests whether one intervention is better than another (e.g., new drug vs. standard care).

b. Non-Inferiority Trial

• Determines whether a new intervention is not worse than an existing treatment by more than a specified margin.

c. Equivalence Trial

• Tests whether two treatments produce similar effects within a predefined range.

d. Pragmatic Trial

• Evaluates interventions under real-world conditions to assess their effectiveness in routine practice.

e. Explanatory Trial

• Conducted in ideal, controlled settings to test an intervention’s biological efficacy.

3. Based on Randomization Method

a. Simple Randomization

• Like flipping a coin—each participant has an equal chance of being in any group.

b. Block Randomization

• Ensures equal-sized groups by randomizing participants in small blocks (e.g., groups of 4 or 6).

c. Stratified Randomization

• Ensures balance within subgroups (e.g., age, sex) before randomization.

d. Adaptive Randomization

• Adjusts randomization probabilities as the trial progresses based on accumulating data.

4. Special Designs

a. Platform Trial

• A flexible design that allows for multiple interventions to be tested within the same study over time.
• Common in COVID-19 and cancer research.

b. Basket Trial

• Tests the same treatment in multiple disease types or subgroups.
• Example: A targeted cancer therapy tested in different tumor types with the same mutation.

c. Umbrella Trial

• Tests multiple treatments in a single disease based on different genetic or molecular characteristics.

Conclusion

The choice of randomized trial depends on study objectives, feasibility, and ethical considerations.

See also: Multi-arm multi-stage trials; Multi-arm trials; Regulated trials; Equity-relevant trials; First-in-man trials; Group sequential trials

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Adapted for educational use. Please cite relevant trial methodology sources when using this material in research or teaching.