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The different types of evidence and their various 'strengths' are often depicted as a pyramid. The pyramid above is from the DePaul University Library's excellent Evidence-Based Nursing Research Guide. In this visual model, the strongest evidence is found at the top, with the weakest found at the bottom. The shape of the pyramid indicates that, in terms of sheer quantity, there are less higher strength sources than weaker sources for most topics. A few things to note:
According to an article in BMJ Journals written by Allison Shorten and Brett Shorten, "Meta-analysis is a research process used to systematically synthesise or merge the findings of single, independent studies, using statistical methods to calculate an overall or ‘absolute’ effect. Meta-analysis does not simply pool data from smaller studies to achieve a larger sample size. Analysts use well recognised, systematic methods to account for differences in sample size, variability (heterogeneity) in study approach and findings (treatment effects) and test how sensitive their results are to their own systematic review protocol (study selection and statistical analysis)."
These can also be known as a clinical practice guideline. In the book Clinical Practice Guidelines We Can Trust, The Institute of Medicine defines them as, "statements that include recommendations, intended to optimize patient care, that are informed by a systematic review of evidence and an assessment of the benefits and harms of alternative care options".
Randomized Controlled Trial
The following definition comes from the article Designing a Research Project: Randomised Controlled Trials and their Principles by JM Kendall: "The randomised control trial (RCT) is a trial in which subjects are randomly assigned to one of two groups: one (the experimental group) receiving the intervention that is being tested, and the other (the comparison group or control) receiving an alternative (conventional) treatment (fig 1). The two groups are then followed up to see if there are any differences between them in outcome. The results and subsequent analysis of the trial are used to assess the effectiveness of the intervention, which is the extent to which a treatment, procedure, or service does patients more good than harm".
Controlled Clinical Trial
Also known as a Non-Randomized Controlled Trial. The PubMed MeSH Heading definition is: "Work consisting of a clinical trial involving one or more test treatments, at least one control treatment, specified outcome measures for evaluating the studied intervention, and a bias-free method for assigning patients to the test treatment. The treatment may be drugs, devices, or procedures studied for diagnostic, therapeutic, or prophylactic effectiveness. Control measures include placebos, active medicine, no-treatment, dosage forms and regimens, historical comparisons, etc.".
Case Control Studies
The PubMed MeSH definition for case control studies is as follows: "Comparisons that start with the identification of persons with the disease or outcome of interest and a control (comparison, referent) group without the disease or outcome of interest. The relationship of an attribute is examined by comparing both groups with regard to the frequency or levels of outcome over time".
Also known as a prospective study. The PubMed MeSH definition is "Studies in which subsets of a defined population are identified. These groups may or may not be exposed to factors hypothesized to influence the probability of the occurrence of a particular disease or other outcome. Cohorts are defined populations which, as a whole, are followed in an attempt to determine distinguishing subgroup characteristics".
This one is a little trickier to define. The best one I could find is from Understanding Qualitative Metasynthesis: Issues and Opportunities in Early Childhood Intervention Research by Elizabeth Erwin, Mary Jane Brotherson, and Jean Ann Summers. "Qualitative metasynthesis is an intentional and coherent approach to analyzing data across qualitative studies. It is a process that enables researchers to identify a specific research question and then search for, select, appraise, summarize, and combine qualitative evidence to address the research question".