Study DesignGlossary Term
ICH E9
A clinical trial design in which participants are randomized to receive one treatment throughout the study, with different groups receiving different treatments simultaneously.
The parallel-group design is the most common clinical trial architecture, in which participants are randomized to receive a single treatment for the duration of their participation while other groups concurrently receive alternative treatments or placebo. Each participant contributes data to only one treatment group, and comparisons are made between the groups rather than within individual participants. This straightforward design is applicable to virtually any therapeutic context and can accommodate any number of treatment arms.
The primary advantage of parallel designs lies in their broad applicability. Unlike crossover studies, parallel trials do not require that participants return to baseline between treatments, making them suitable for diseases that progress, for treatments with lasting effects, and for acute conditions. They avoid the complications of carryover effects and do not require extended study duration to accommodate multiple treatment periods. The design is conceptually simple and well-understood by regulators, clinicians, and statisticians.
The parallel design does require larger sample sizes compared to crossover studies because between-subject variability contributes to the overall variance in treatment comparisons. This limitation can be partially addressed through stratified randomization, which ensures balance on key prognostic factors, or through covariate adjustment in the analysis. The design is also susceptible to differential dropout if participants in one treatment group discontinue at higher rates, potentially introducing bias. Nevertheless, the parallel design remains the workhorse of clinical development, suitable for most Phase II and Phase III trials across therapeutic areas.
Phase III efficacy trial
"The pivotal Phase III trial employed a two-arm parallel design with participants randomized 2:1 to receive either the investigational therapy or matching placebo for 24 weeks."
Dose-finding study
"The Phase II study used a four-arm parallel design comparing three dose levels of the investigational drug against placebo to identify the optimal dose for Phase III development."
The fraction of an administered dose of a drug that reaches the systemic circulation unchanged, and the rate at which this occurs.
A group of participants in a clinical trial who receive a comparator treatment, placebo, or no treatment to serve as a baseline for evaluating the effects of the investigational intervention.
A clinical trial design in which participants receive multiple treatments in sequence, with each participant serving as their own control by receiving all study treatments during different periods.
A systematic approach to increasing the dose of an investigational product during a clinical trial, typically employed in early-phase studies to identify safe and potentially effective dose levels.
A clinical trial in which both the participants and the investigators are unaware of the treatment assignments, providing maximal protection against bias in the conduct and assessment of the study.
ICH E9
A clinical trial design in which participants are randomized to receive one treatment throughout the study, with different groups receiving different treatments simultaneously.
The parallel-group design is the most common clinical trial architecture, in which participants are randomized to receive a single treatment for the duration of their participation while other groups concurrently receive alternative treatments or placebo. Each participant contributes data to only one treatment group, and comparisons are made between the groups rather than within individual participants. This straightforward design is applicable to virtually any therapeutic context and can accommodate any number of treatment arms.
The primary advantage of parallel designs lies in their broad applicability. Unlike crossover studies, parallel trials do not require that participants return to baseline between treatments, making them suitable for diseases that progress, for treatments with lasting effects, and for acute conditions. They avoid the complications of carryover effects and do not require extended study duration to accommodate multiple treatment periods. The design is conceptually simple and well-understood by regulators, clinicians, and statisticians.
The parallel design does require larger sample sizes compared to crossover studies because between-subject variability contributes to the overall variance in treatment comparisons. This limitation can be partially addressed through stratified randomization, which ensures balance on key prognostic factors, or through covariate adjustment in the analysis. The design is also susceptible to differential dropout if participants in one treatment group discontinue at higher rates, potentially introducing bias. Nevertheless, the parallel design remains the workhorse of clinical development, suitable for most Phase II and Phase III trials across therapeutic areas.
Phase III efficacy trial
"The pivotal Phase III trial employed a two-arm parallel design with participants randomized 2:1 to receive either the investigational therapy or matching placebo for 24 weeks."
Dose-finding study
"The Phase II study used a four-arm parallel design comparing three dose levels of the investigational drug against placebo to identify the optimal dose for Phase III development."
The fraction of an administered dose of a drug that reaches the systemic circulation unchanged, and the rate at which this occurs.
A group of participants in a clinical trial who receive a comparator treatment, placebo, or no treatment to serve as a baseline for evaluating the effects of the investigational intervention.
A clinical trial design in which participants receive multiple treatments in sequence, with each participant serving as their own control by receiving all study treatments during different periods.
A systematic approach to increasing the dose of an investigational product during a clinical trial, typically employed in early-phase studies to identify safe and potentially effective dose levels.
A clinical trial in which both the participants and the investigators are unaware of the treatment assignments, providing maximal protection against bias in the conduct and assessment of the study.