Let’s say you are still working to solve the issue of whether gum chewing reduces post-operative ileus. You identified titles of all relevant articles using PubMed database (http://www.ncbi.nlm.nih.gov/pubmed), and had the librarian pull the full articles for you.
Professionals have agreed on which types of evidence are strongest. Here’s one well-accepted hierarchy form strongest #1 to weakest #6 (Melnyk & Fineout-Overholt, 2005).
- Systematic reviews, Meta-analyses, or Evidence-based clinical practice guidelines based on systematic review of RCTs
- Randomized controlled trial
- Controlled trials without randomization
- Case controlled or cohort studies
- Systematic review of descriptive studies
- Single descriptive or qualitative study
- Expert opinion of individuals or committees
Number 1= Strongest. Number 6=Weakest
When you are trying to solve a problem, FIRST look for the three (3) types of evidence that are the very strongest (#1). These are:
- Systematic reviews that are summaries of research findings from many studies;
- Meta-analyses that are summaries of research findings in which the data from those other studies are combined into one big study;
- Evidence-based clinical practice guidelines that are clinical recommendations based on a summary of research and other evidence. An expert panel has often agreed on the summary and recommendations.
Your next strongest option, #2, is at least one randomized controlled trial (RCT). In an RCT a group of subjects is randomly separated into at least two groups. One group gets the experimental treatment—whether it is a drug or teaching plan or something else—and the other group usually gets standard treatment or a placebo. Then the group outcomes are compared statistically to see which did better.
Usually the title or first few lines of the article will tell you that the article is a systematic review, a meta-analysis, an evidence-based clinical practice guideline, or an RCT. Rarely is this left a mystery! (Never assume that a research study article is strong just because you LIKE the findings, or that it is weak because you DON’T like the findings.)
I’ll comment on other levels of evidence soon, but let’s focus on the strongest types first. Try the critical thinking for practicing the ideas above.
Critical Thinking: Using the article titles below, rank these three (3) research studies in order from Strongest evidence to Weakest evidence:
- Gum chewing reduces the time to first defaecation after pelvic surgery: A randomised controlled study.
- Chewing gum and postoperative ileus in adults: A systematic literature review and meta-analysis.
- Does gum chewing help prevent impaired gastric motility in the postoperative period?
Want to read more? A good summary of one hierarchy is and why it’s important is at this 5 minute youtube video: https://www.youtube.com/watch?v=5H8w68sr0u8 . While that hierarchy does not precisely match the one above, the video still has lots of good information.