Chart — RT Career & Professional Practice
Levels of Evidence in Respiratory Care
Not all evidence carries equal weight. This chart ranks study designs from strongest to weakest, with what each one can tell you and where it falls short — the backbone of evidence-based practice.
Written by Apex Respiratory Editorial Team
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Hierarchy of Evidence
| Level | Study Design | What It Shows | Limitation |
|---|---|---|---|
| 1 (highest) | Systematic review or meta-analysis of RCTs | Pooled, critically appraised results across trials | Only as good as the included studies |
| 2 | Randomized controlled trial (RCT) | Cause and effect, with randomization controlling bias | Cost, feasibility, and external validity |
| 3 | Cohort study | Outcomes over time in exposed versus unexposed groups | Confounding; not randomized |
| 4 | Case-control study | Compares those with and without an outcome, looking backward | Recall and selection bias |
| 5 | Case series or case report | Describes a group of patients or a single case | No control group; not generalizable |
| 6 (lowest) | Expert opinion or bench research | Consensus or mechanistic reasoning | Subject to bias; not direct clinical evidence |
How to Use This Chart
Understanding where a study falls in the evidence hierarchy helps clinicians weigh findings appropriately when making patient care decisions or interpreting clinical practice guidelines.
- Higher levels control bias better. A systematic review of RCTs is the strongest single source because it pools results across multiple well-designed trials.
- Quality matters as much as design. A well-designed lower-level study can still outrank a poorly done higher-level one; appraise quality, not just design.
- AARC Clinical Practice Guidelines grade their recommendations using an evidence hierarchy like this one, so familiarity with these levels is essential for understanding respiratory care CPGs.
Related Resources
Sources
- Guyatt G, Rennie D, Meade MO, Cook DJ. Users' Guides to the Medical Literature: A Manual for Evidence-Based Clinical Practice. 3rd ed. McGraw-Hill; 2015.
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021. Quality and evidence-based respiratory care.