(From BMJ’s Clinical Evidence Glossary)
Meta-analysis: A statistical technique that summarizes the results of several studies in a single weighted estimate, in which more weight is given to results of studies with more events and sometimes to studies of higher quality.
Systematic Review: a review in which specified and appropriate methods have been used to identify, appraise, and summarize studies addressing a defined question. (It can, but need not, involve meta-analysis). In Clinical Evidence, the term systematic review refers to a systematic review of RCTs unless specified otherwise.
Randomized Controlled Trial: a trial in which participants are randomly assigned to two or more groups: at least one (the experimental group) receiving an intervention that is being tested and another (the comparison or control group) receiving an alternative treatment or placebo. This design allows assessment of the relative effects of interventions.
Controlled Clinical Trial: a trial in which participants are assigned to two or more different treatment groups. In Clinical Evidence, we use the term to refer to controlled trials in which treatment is assigned by a method other than random allocation. When the method of allocation is by random selection, the study is referred to as a randomized controlled trial (RCT). Non-randomized controlled trials are more likely to suffer from bias than RCTs.
Cohort Study: a non-experimental study design that follows a group of people (a cohort), and then looks at how events differ among people within the group. A study that examines a cohort, which differs in respect to exposure to some suspected risk factor (e.g. smoking), is useful for trying to ascertain whether exposure is likely to cause specified events (e.g. lung cancer). Prospective cohort studies (which track participants forward in time) are more reliable than retrospective cohort studies.
Case control study: a study design that examines a group of people who have experienced an event (usually an adverse event) and a group of people who have not experienced the same event, and looks at how exposure to suspect (usually noxious) agents differed between the two groups. This type of study design is most useful for trying to ascertain the cause of rare events, such as rare cancers.
Case Series: analysis of series of people with the disease (there is no comparison group in case series).
Qualitative Studies:* Research that involves the collection of data in nonnumeric form, such as personal interviews, usually with the intention of describing a phenomenon.
Quantitative Studies:* Research that collects data in numeric form and emphasizes precise measurement of variables; often conducted in the form of rigorously controlled studies.
Quasi-experiments:* A type of experimental design that tests the effects of an intervention or treatment but lacks one or more characteristics of a true experiment (e.g. random assignment; a control or comparison group).
(*Melnyk, B. M., & Fineout-Overholt, E. (2005). Evidence-based practice in nursing & healthcare :A guide to best practice. Philadelphia: Lippincott Williams & Wilkins.)
Questions related to the care of a patient typically fall into defined clinical scenarios or clinical categories. You will want to identify which category your research question falls under when searching for evidence based information. Identifying the clinical scenario helps you understand which types of studies to look for in a database. Some databases like CINAHL and PubMed MEDLINE even have filters that allow you to limit your search according to your clinical scenario! That saves you time and energy.
Common scenarios include: Therapy (Intervention), Diagnosis, Etiology/Harm, Prognosis, and Meaning
Melnyk and Fineout-Overholt (2015) provide guidance on which types of studies best address these scenarios. The studies are listed in order of strength, with the strongest source of evidence listed first (level 1). Synthesis or compilations of multiple studies typically provide the best evidence and are always level 1.
| Type of Question | Type of Study Methodology |
| All Clinical Questions | Systematic review, meta-analysis |
| Therapy (Intervention) or Diagnosis | 1. Systematic review or meta-analysis of randomized controlled trials (RCTs) 2. RCTs 3. Non-RCTs 4. Cohort studies or case-control studies 5. Meta-systhesis of qualitative or descriptive studies 6. Qualitative or descriptive single studies 7. Expert opinion |
| Etiology/Harm or Prognosis | 1. Synthesis of cohort study or case-control studies 2. Single cohort study or case-control studies 3. Meta-synthesis of qualitative or descriptive studies 4. Single qualitative or descriptive studies 5. Expert opinion |
| Meaning | 1. Meta-synthesis of qualitative studies 2. Single qualitative studies 3. Synthesis of descriptive studies 4. Single descriptive studies 5. Expert opinion |
| Quality Improvement |
1. Systematic review or meta-analysis of randomized controlled trials (RCTs) |