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Kornhauser Health Sciences Library

Evidence-Based Practice: Types of Evidence

Types of Research

Once you have your focused question, it's time to decide on the type of evidence you need to answer it. Understanding the types of research will help guide you to proper evidence that will support your question.

Primary Research

Secondary Research

The data that is obtained during a study that has been conducted. Examples include surveys, information from focus groups, questionnaires, and the like. Primary research presented in the results of randomized controlled trials and observational studies.

The summary and analysis of already existing research. Examples include systematic reviews, meta-analyses, review articles, and textbooks.

Qualitative Research

Quantitative Research

Hypothesis generating.

Collecting and analyzing non-numerical data to understand concepts, opinions, or experiences.

Hypothesis testing.

Collecting and analyzing numerical data to describe characteristics and find correlations.

Evidence Based Pyramid



Pyramid Logo

As you move up the pyramid, the study designs are more rigorous and are less biased.

What type of study should you use?

Question Definitions:

Intervention/Therapy: Questions addressing the treatment of an illness or disability.

Etiology: Questions addressing the causes or origins of disease (i.e., factors that produce or predispose toward a certain disease or disorder).

Diagnosis: Questions addressing the act or process of identifying or determining the nature and cause of a disease or injury through evaluation.

Prognosis/Prediction: Questions addressing the prediction of the course of a disease.

The type of question you have will often lead you to the type of research that will best answer the question:

Intervention/Prevention:  RCT > Cohort Study > Case Control > Case Series

Therapy:  RCT > Cohort > Case Control > Case Series

Prognosis/Prediction:  Cohort Study > Case Control > Case Series

Diagnosis/Diagnostic:  Prospective, blind comparison to Gold Standard

Etiology:  RCT > Cohort Study > Case Control > Case Series


Table of Definitions
Category Definition
Case Reports or Case Studies Detailed reports or collections of reports for individual patients.
Case Control Studies Studies in which patients who currently have a condition are compared to those who do not. Researchers look back retrospectively to identify possible exposures. Case control studies are less reliable than randomized control trials because of the lack of ability to demonstrate causal relationships.
Cohort Studies Identifies and follows two groups of patients, one group who has received an exposure of interest and one who has not.
Randomized Control Trial (RCT) A scientific study that randomly assigns participants to a test or control group. Primary data is generated and outcomes are compared between the two groups.
Systematic Review A review of all the available literature on a specific topic that has been systematically searched for, critically appraised, and summarized; usually focused on a answering a specific clinical question
Meta-Analysis Taking a systematic review a step further and applying a quantitative statistical analysis to the methodology used in the included literature to test for statistical significance.
Clinical Practice Guidelines Provide steps and tools for appropriate diagnosis and treatment; based on an examination of current evidence within the paradigm of evidence-based medicine.

CEBM Study Design Tree

Flow-chart depicting study design

The type of study can generally be worked at by looking at three issues:

Q1. What was the aim of the study?

  1. To simply describe a population (PO questions) descriptive
  2. To quantify the relationship between factors (PICO questions) analytic.

Q2. If analytic, was the intervention randomly allocated?

  1. Yes? RCT
  2. No? Observational study

For observational study the main types will then depend on the timing of the measurement of outcome, so our third question is:

Q3. When were the outcomes determined?

  1. Some time after the exposure or intervention? cohort study (‘prospective study’)
  2. At the same time as the exposure or intervention? cross sectional study or survey
  3. Before the exposure was determined? case-control study (‘retrospective study’ based on recall of the exposure)

from Centre for Evidence-Based Medicine