In the sciences, information is often categorized into three types of sources: primary, secondary and tertiary. These source types are determined based on how far removed the information in the source is from original research.
In the sciences, a primary source is a source that includes original research that's documented at the time of study. A primary source will generally have a Methods section that describes the researchers' procedures and materials. Primary sources can take the form of:
Secondary sources offer analysis, evaluation, interpretation and/or synthesis of primary sources. A secondary source could be:
Tertiary sources offer summaries or compilations of information from primary and secondary sources. A tertiary source could be:
Systematic Reviews and Meta-Analyses are generally considered to be the highest quality of evidence. But the type of study you need may differ based on what type of research question you are trying to answer. You'll want to identify which are the most valid research designs that can "prove" the research question with the understanding that there may not be much research done on that research question.
Clinical Question |
Suggested Research Design(s) |
All Clinical Questions |
Systematic review, meta-analysis |
Therapy |
Randomized controlled trial (RCT), meta-analysis |
Etiology |
Randomized controlled trial (RCT), meta-analysis, cohort study |
Diagnosis |
Randomized controlled trial (RCT) |
Prevention |
Randomized controlled trial (RCT), meta-analysis |
Prognosis |
Cohort study Also: case-control study, case series |
Meaning |
Qualitative study |
Quality Improvement |
Randomized controlled trial (RCT) |
Cost |
Economic evaluation |
Table source: OHSU's Evidence-Based Practice Toolkit for Nursing