Levels of evidence
The pyramid is an appropriate shape for this graphic, as it represents the quality of research designs by level, as well as the quantity of each study design in the body of published literature. Systematic reviews (higher quality), for instance, are the most time-intensive articles to write and are therefore rarer (lower quantity) than other types of studies.
Though finding research studies high on the pyramid is preferred, Evidence-Based Nursing may need to draw on research designs lower in the evidence hierarchy than case series. Occasionally nothing but case reports or even bench research may exist on a topic. When making evidence-based decisions for patient care, it is essential to select the highest level research design available for the specific question of interest.
A summary of the medical literature that uses explicit methods to perform a comprehensive literature search and critical appraisal of individual studies, and that uses appropriate statistical techniques to combine these valid studies.
A meta-analysis is a particular type of systematic review that attempts to combine and summarize quantitative data from multiple studies using sophisticated statistical methodology. Such a strategy strengthens evidence as it makes the small sample size of individual studies much larger, giving the results more statistical power and, therefore, more credibility than the individual studies. Meta-analyses are not comprehensive, as only compatible data may be combined into a larger data set.
A randomized controlled trial is an experimental, prospective study in which "participants are randomly allocated into an experimental group or a control group and followed over time for the variables/outcomes of interest." 
Study participants are randomly assigned to ensure that each participant has an equal chance of being assigned to an experimental or control group, thereby reducing potential bias. Outcomes of interest may be death (mortality), a specific disease state (morbidity), or even a numerical measurement such as blood chemistry level.
A cohort study is an observational, prospective or retrospective study. A cohort study "involves identification of two groups (cohorts) of patients, one that received the exposure of interest, and one that did not, and following these cohorts forward for the outcome of interest." 
While at first glance a cohort study looks similar to a RCT, it differs in one very significant way: the researchers do not assign the exposure or randomize the groups in any way. RCTs are experimental, while cohort studies are observational.
A case-control study is an observational, retrospective study which "involves identifying patients who have the outcome of interest (cases) and control patients without the same outcome, and looking back to see if they had the exposure of interest."
A case series is a descriptive report "on a series of patients with an outcome of interest. No control group is involved.”
The most common type of questions related to clinical tasks and suggested best type of study
|Diagnosis||Prospective, blind comparison to gold standard|
|Therapy/Prevention||RCT>cohort>case control>case series|
|Prognosis||RCT>case control>case series|
|Causation/Harm/Etiology||RCT>cohort>case control >case series|
MEDLINE has only two thesaurus-based options - the recently
introduced term ‘Qualitative Research’ or the more established ‘Nursing