| 2012 American Association of Critical-Care Nurses levels of evidence with revisions to 2008 hierarchy | ||
| Category | Level | Description |
| Experimental evidence | A | Meta-analysis or meta synthesis of multiple controlled studies with results that consistently support a specific action, intervention, or treatment (systematic review of a randomized controlled trial) |
| B | Evidence from well-designed controlled studies, both randomized and nonrandomized, with results that consistently support a specific action, intervention, or treatment | |
| C | Evidence from qualitative, integrative reviews, or systematic reviews of qualitative, descriptive, or correlational studies or randomized controlled trials with inconsistent results | |
| Recommendations | D | Evidence from peer-reviewed professional organizational standards, with clinical studies to support recommendations |
| E | Theory-based evidence from expert opinion or multiple case reports | |
| M | Manufacturer’s recommendation only | |
Sources
Armola RR, Bourgault AM, Halm MA, Board RM, Bucher L, Harrington L, Heafey CA, Lee R, Shellner PK, Medina J. (2009) AACN levels of evidence: what's new? J.Crit Care Nurse. Aug;29(4):70-3.
Peterson et. al. (2014). AACN Evidence Leveling System, Retrieved from https://pdfs.semanticscholar.org/67c4/545a68c7746af4a47b70d0e8f60a4e2fc25d.pdf
How to Read a Paper: the basics of evidence-based medicine by 
Users' Guides to the Medical Literature: a Manual for Evidence-Based Clinical Practice, 3E by 
Nursing Research: methods and critical appraisal for evidence-based practice by 
Reading Research by The critical appraisal process hinges on three questions that apply to any study:
1. Are the results of the study valid? (Validity)
2. What are the results? (Reliability)
3. Will the results help me in caring for my patients? (Applicability)
4. When was the article published? Is the information out of date for your purposes? (Currency)
This process provides clinicians with the tools to interpret the quality of studies and determine the applicability of the synthesis of multiple studies' results to their patients.
The validity of a study refers to whether the results of the study were obtained via sound scientific methods. Bias (defined as the systematic deviation from the truth) and/or confounding variables may compromise the validity of the finding. The reliability of the study finds were determined by the size of the intervention's effect (the effect size) and how precisely that effect was estimated. This part of critical appraisal examined the numerical data reported in the results section of a study. When critically appraising the the usefulness of a study for clinical decision making, a basic aspect of applicability is to evaluate the study patients in comparison with the patients to whom the evidence would be applied.