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Shauni FitzgeraldMay 17, 2024 1:58:44 PM5 min read

Optimizing Study Design: Selecting Primary and Secondary Outcomes

Optimizing Clinical Study Design: The Art of Selecting Primary and Secondary Outcomes

Designing a successful clinical study requires careful consideration of your study objectives and endpoints. In this blog, we'll dive into the intricacies of selecting primary and secondary outcomes, exploring regulatory best practices for EFSA health claims and the importance of a well-defined statistical analysis plan.

Table of Contents

Understanding Study Objectives and Endpoints

Study objectives and endpoints are foundational elements in clinical research. They guide the study design and provide a framework for analysis.

Defining Study Objectives

Study objectives address the broad question posed by the scientific inquiry. They outline the goals of the research and the hypotheses being tested.

Understanding Endpoints

Endpoints are the quantitative measurements required to address study objectives. They provide specific data points for analysis.

Classifying Objectives: Primary, Secondary, and Exploratory

Objectives in clinical studies are classified into three main categories: primary, secondary, and exploratory. Each category serves a distinct purpose and requires different levels of predefinition.

Primary Objectives

Primary objectives represent the most crucial outcomes of a study. They are the main focus and are often used to determine the study's success.

Secondary Objectives

Secondary objectives provide additional information that supports the primary outcomes. They help to understand broader effects and validate the primary findings.

Exploratory Objectives

Exploratory objectives offer flexibility and allow researchers to investigate additional questions. They are less rigidly defined and can adapt as the study progresses.

The Primacy of the Primary Objective

The primary objective holds the highest importance in a clinical study. It is the cornerstone around which the study is designed and analyzed.

Regulatory Perspective

From a regulatory viewpoint, the primary objective is critical. Most studies are designed with a single primary objective to maintain clarity and focus.

Statistical Power

The sample size of a study is often determined based on the primary objective. It must have sufficient statistical power to detect the effect of the intervention.

Handling Multiple Primary Objectives

While best practice suggests having a single primary objective, some studies may include more than one. In such cases, careful consideration is required to ensure each objective is adequately powered.

The Role of Secondary Objectives

Secondary objectives play a crucial role in enhancing the understanding of the primary outcomes in a clinical study. They offer additional insights that support the primary objective, helping to paint a more comprehensive picture of the intervention's effects.

Supplementing Primary Outcomes

Secondary objectives are designed to provide additional context to the primary outcomes. For example, in a blood pressure study, while the primary outcome might be the change in systolic blood pressure, a secondary outcome could be the change in diastolic blood pressure.

Regulatory Considerations

Although secondary objectives are important, they have less regulatory weight compared to primary objectives. However, they still need to be part of a pre-specified analysis plan to ensure their credibility and relevance.

Limiting the Number of Secondary Objectives

It's essential to limit the number of secondary objectives to avoid overcomplicating the study. A study with too many secondary endpoints can be criticized and may lose focus.

Anecdotal Evidence

There have been instances where studies with an excessive number of secondary endpoints faced criticism. For example, a study with 18 secondary endpoints was deemed not credible by EFSA's NDA panel.

Exploratory Objectives and Physiological Biomarkers

Exploratory objectives offer the flexibility to investigate additional questions and generate new hypotheses. These objectives often include physiological biomarkers that may not show a direct treatment effect but support the potential mode of action of the product.

Flexibility in Study Design

Exploratory objectives are less rigidly defined, allowing researchers to adapt and explore new avenues as the study progresses. This flexibility makes them ideal for generating new hypotheses.

Role of Physiological Biomarkers

Physiological biomarkers are often included as exploratory endpoints. While they may not directly indicate a treatment effect, they provide valuable information about the potential mode of action of the product.

When to Use Exploratory Objectives

Exploratory objectives are particularly useful when researchers want to investigate additional questions that arise during the study. They allow for greater leeway and can be adjusted as new data emerges.

Guidance from EFSA on Appropriate Endpoints

The European Food Safety Authority (EFSA) provides detailed guidance on the scientific requirements for health claims. These guidelines outline acceptable clinical and biomarker outcomes for substantiating health claims.

Published Guidance Documents

EFSA has published six guidance documents that outline the acceptable clinical and biomarker outcomes for health claims. These documents are based on submissions since 2007.

Utility of Guidance

Some guidance documents are more useful than others, depending on the specific health claim being investigated. It's essential to refer to the most up-to-date guidelines.

Updating Guidelines

EFSA continually updates its guidelines to reflect new scientific evidence and regulatory requirements. Some of the older documents date back to 2012 and may have been revised.


Key Takeaways

  1. Defining Objectives and Endpoints: Clear study objectives and endpoints are essential for robust clinical study design and analysis.
  2. Primary Objectives: These are the cornerstone of a study, determining its focus, regulatory importance, and statistical power.
  3. Secondary Objectives: They provide additional context to the primary outcomes but should be limited to avoid overcomplication.
  4. Exploratory Objectives: Offer flexibility for investigating additional questions and often include physiological biomarkers.
  5. EFSA Guidelines: EFSA provides detailed guidance on acceptable clinical and biomarker outcomes, continually updated to reflect new scientific evidence.


Here are some common questions about selecting primary and secondary objectives in clinical studies.

Why is it crucial to define primary and secondary objectives in advance?

Defining these objectives early ensures a robust study design and sets a clear path for statistical analysis.

How do primary objectives impact the study design?

Primary objectives determine the study's focus and its overall success, influencing sample size and statistical power.

What role do secondary objectives play?

Secondary objectives provide additional context and support to the primary outcomes, enhancing the study's overall findings.

Can a study have multiple primary objectives?

While it's best to have a single primary objective, some studies may include more. Each must be adequately powered to maintain credibility.


Shauni Fitzgerald

Shauni Fitzgerald, a seasoned Regulatory Affairs & Research Manager at Atlantia Clinical Trials, excels in clinical study design, regulatory submissions, and quality management. With a master's in Nutritional Sciences, she brings expertise and leadership to the field.