Classify the Articles and Reports by Type of Research Design
  Research Design Algorithm

Classify the Articles and Reports by Type of Research Design

Which type of study is preferred?

The four most common typese of evidence analysis questions are: diagnosis, treatment, prognosis and etiology. The type of question you are trying to answer determines the best research design to seek.

For instance, a randomized controlled trial (RCT) would be the most appropriate type of study to answer a question about therapy or treatment. This hierarchy is often shown graphically as a pyramid with expert opinions at the bottom of the pyramid and randomized controlled trials (RCTs) at the top.

However, a RCT would not be the strongest research design to answer a question about prognosis. The hightest level of evidence for prognosis is a cohort study. always look for the strong evidence you can find to answer your type of question.

In some situations the eligibility of a research article depends on the research design used. For example, in questions about the effectiveness of a treatment or intervention, a randomized controlled trial is the preferred research design; however, questions about etiology, causation or harm are best answered with cohort or case control research designs; diagnosis and screening questions can be answered with cross-sectional designs; and natural history and prognosis questions use cohort designs. There might not be much research available for new and emerging areas of practice or for practices that historically have been accepted as usual practice. In these situations, which are common in dietetics, all research designs are included but greater weight is given to results from studies using designs that best answer the research question.

First, divide the studies and reports listed on the Search Plan & Results template into two categories: primary research (original studies) and secondary research (review, meta-analysis and/or syntheses of previously reported studies).

Second, classify the studies or reports according to the type of research, that is, by study design. Study designs are organized into a hierarchy based on the ability of the design to test causal relationships.Below is a table of classification system used by the Academy. A glossary of research terms are presented in Appendix 6. The type of research design is determined during the critical appraisal step and recorded on the quality checklist template.

Hierarchy and Classification of Studies2

Primary Reports

Secondary Reports


Randomized controlled trial (RCT)

Cluster Randomized Trial

Randomized Crossover Trial


Meta-analysis or Systematic review

Decision analysis

Cost-benefit analysis

Cost-effectiveness study


Prospective Cohort Study

Retrospective Cohort Study


Non-Randomized Controlled Trial

Non-Randomized Crossover Trial

Case-Control Study

Time Series Study

Diagnostic, Validity or Reliability Study


Narrative review (Review article)

Consensus statement

Consensus report


NOn-Controlled Trial

Case Study or Case Series

Other Descriptive Study

Cross-Sectional Study

Trend Study

Before-After Study


Medical opinion

Classifying studies and reports gives an initial picture of the type of studies and level of evidence available. It also helps organize the articles for the next step of critical appraisal. The Academy uses a study design algroithm to help you identify the study design. This classification is then recorded on the article's worksheet template.

The Academy web-based presentation "Research Designs and Threats to Validity" is a helpful tool to assist with the classification of research studies.

2Adapted by the Academy of Nutrition and Dietetics from ©Joint Commission Resources: "A Practical Approach to Evidence Grading." Joint Commission Journal on Quality Improvement 2000:Volume 26(12):707

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