Got a clinical problem? You probably want to solve it with evidence—STRONG evidence. Click on this link to see one well-accepted hierarchy from strongest #1 to weakest #7 (Melnyk & Fineout-Overholt, 2005). Today let’s look at the 4th strongest level of evidence = Case controlled or cohort studies
First a quick review
Click here for a quick review of the strongest 2 levels of evidence (#1 Systematic reviews, Meta-analyses, or Evidence-based clinical practice guidelines based on systematic review of RCTs. #2 Randomized controlled trials)
Click here for a review of the 3rd strongest type of evidence (#3Controlled trials without randomization)
All 3 of the top, strongest levels of evidence are experimental studies (or include available experimental studies). That means the researcher actually does something or gives a treatment to some of the subjects and then records the outcomes.
The weaker 4 levels of evidence are non-experimental designs. This means that the researcher merely observes & does Not do anything to subjects. So how does that work?!
First, a cohort study (non-experimental). A cohort study starts with a group of people who have something in common and then the researcher observes only & keeps collecting data from them over a long time into the future. Data collection into the future is called a prospective study. “An example is the Nurses’ Health Study, in which over 20,000 nurses were identified and followed-up annually with tests and surveys for over 25 years (this study is still ongoing). These studies provide very valuable information, but are obviously very expensive and time-consuming.”(OMERAD EBM course, 2008)
Now a case-controlled study (non-experimental). In a case controlled study the researcher observes only & collects data over time into the past (not the future). Data collection into the past is called a retrospective study. Again, from the OMERAD EBM (2008) site this example: “Patients with a disease are identified who have suffered a bad outcome such as death or recurrence, and compared with patients who have the disease but haven’t suffered the bad outcome. For example, a researcher might identify a group of breast cancer patients who have died…, and compare them with a similar group of patients with breast cancer who are still living.”
- You are a runner in the Los Angeles marathon and you are interested in how that race can improve cardiovascular health among those who finish. Question: Cohort or Case controlled?
- Some finishers of the LA marathon die of heart attacks 20 years later; many survive another 40 years. Question: Cohort or Case controlled?
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