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Research Guides@Tufts

NUTR202 Principles of Nutrition Science


This section will introduce you to the following study designs frequently used in the health sciences. Each box in this section describes a particular study design and will include a definition of the design and why it is used. For Information on how to retrieve specific study designs on PubMed, please see Retrieving Articles by Study Design in PubMed.

Clinical Trials

What is a clinical trial?

“[A] clinical trial is any research study that prospectively assigns human participants or groups of humans to one or more health-related interventions to evaluate the effects on health outcomes. Clinical trials may also be referred to as interventional trials. Interventions include but are not restricted to drugs, cells and other biological products, surgical procedures, radiologic procedures, devices, behavioural treatments, process-of-care changes, preventive care, etc. This definition includes Phase I to Phase IV trials.”[1] 

Trial Phases [1]:

I.  Clinical trials test a new biomedical intervention in a small group of people (e.g., 20-80) for the first time to evaluate safety (e.g., to determine a safe dosage range and to identify side effects).

II. Clinical trials study the biomedical or behavioral intervention in a larger group of people (several hundred) to determine efficacy and to further evaluate its safety.

III. Studies investigate the efficacy of the biomedical or behavioral intervention in large groups of human subjects (from several hundred to several thousand) by comparing the intervention to other standard or experimental interventions as well as to monitor adverse effects, and to collect information that will allow the intervention to be used safely.

IV. Studies are conducted after the intervention has been marketed. These studies are designed to monitor effectiveness of the approved intervention in the general population and to collect information about any adverse effects associated with widespread use.

Why use this study type?

  • Evaluating one or more interventions (for example, drugs, medical devices, approaches to surgery or radiation therapy) for treating a disease, syndrome, or condition. [2]
  • Finding ways to prevent the initial development or recurrence of a disease or condition. These can include medicines, vaccines, or lifestyle changes, among other approaches. [2]
  • Evaluating one or more interventions aimed at identifying or diagnosing a particular disease or condition.[2]
  • Examining methods for identifying a condition or risk factors for that condition.[2]
  • Exploring and measuring ways to improve the comfort and quality of life of people with a chronic illness through supportive care.[2]

Format and features


  1. WHO International Clinical Trials Registry Platform Glossary
  3. UCSF Clinical trial Protocol Development
  4. CDC TREND Statement Checklist
  5. The CONSORT Group

Cohort Study

What is a cohort study?

"A cohort study tracks two or more groups forward from exposure to outcome. This type of study can be done by going ahead in time from the present (prospective cohort study) or, alternatively, by going back in time to comprise the cohorts and following them up to the present (retrospective cohort study)" [1]

Why use this type of study?

  • To identify incidence and natural history of a disease [1] 
  • To examine multiple outcomes after a single exposure [1] 
  • As a substitution for  an experiment when experimentation is not available [2]

Format and features


  1. Grimes DA, Schulz KF. Cohort studies: marching towards outcomes. The Lancet. 2002;359(9303):341-345.
  2. Chapter 5: Risk: Looking forward. In: Fletcher RH, Fletcher SW, eds. Clinical epidemiology : the essentials. 4th ed. Baltimore: Lippincott Williams & Wilkins; 2005: p. 85.
  3. Strengthening the Reporting of Observational Studies in Epidemiology (STROBE). STROBE checklist for cohort studies. [2007]; Accessed March 1, 2013.

Randomized Controlled Trial (RCT)

What is a randomized controlled trial (RCT)?

“A study design that randomly assigns participants into an experimental group or a control group. As the study is conducted, the only expected difference between the control and experimental groups in a randomized controlled trial (RCT) is the outcome variable being studied.”[1]


Why use this type of study?

  • To reduces bias [2]
  • To approximate a controlled experiment.[2]
  • Statistically efficient [2]

Format and features


  1. Himmelfarb Health Sciences Library (The George Washington University). Study Design 101: Randomized Controlled Trial. 2011; Accessed January 30, 2012.
  2. Grimes DA, Schulz KF. An overview of clinical research: the lay of the land. The Lancet. 1/5/ 2002;359(9300):57-61.
  3. The CONSORT Group. CONSORT 2010 checklist of information to include when reporting a randomised trial. 2010; Accessed January 31, 2013.

Systematic Review and Meta-Analysis


What is a systematic review/meta-analysis?

 “A systematic review is a review of a clearly formulated question that uses systematic and explicit methods to identify, select, and critically appraise relevant research, and to collect and analyze data from the studies that are included in the review. Statistical methods (meta-analysis) may or may not be used to analyze and summarize the results of the included studies. [1]

 “Meta-analysis refers to the use of statistical techniques in a systematic review to integrate the results of included studies.”[1]


Why use this type of study?

  •   “Systematic reviews establish whether scientific findings are consistent and can be generalized across populations, settings, and treatment variations, or whether findings vary significantly by particular subsets.” [2]
  •   “ Meta-analyses in particular can increase power and precision of estimates of treatment effects and exposure risks.” [2]
  • “Explicit methods used in systematic reviews limit bias…[which improves] reliability and accuracy of conclusions.”[2]


Format and features



  1. The PRISMA Group. The PRISMA Statement. Accessed January 31, 2013.
  2. Mulrow CD. Systematic Reviews: Rationale for systematic reviews. BMJ. 1994-09-03 08:00:00 1994;309(6954):597-599.
  3. The PRISMA Group. PRISMA 2009 Checklist. Accessed January 31, 2013.

See also

Systematic Reviews LibGuide



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