Systematic reviews and meta-analyses are situated at the top of what is known as the “Evidence Pyramid” (see figure below). As you move up the pyramid the amount of available literature on a given topic decreases, but the relevancy and quality of that literature increases. Systematic reviews and meta-analyses are considered to be the highest quality evidence on a research topic because their study design reduces bias and produces more reliable findings. However, you may not always be able to find (or need to find) the highest level of evidence to answer your research question. In the absence of the best evidence, you then need to consider moving down the pyramid.
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TYPES OF DESIGN Studies:
Clinical Practice Guidelines (CGP) - The Institute of Medicine (IOM) defines clinical practice guidelines 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."
A Meta-analysis takes a systematic review one step further by combining all the results using accepted statistical methodology.
Systematic Reviews usually focuses on a specific clinical question and conducts an extensive literature search to identify studies with sound methodology. The studies are reviewed, assessed, and the results summarized according to the predetermined criteria of the review question.
Randomized, controlled clinical trials. A prospective, analytical, experimental study using primary data generated in the clinical environment. Individuals similar at the beginning are randomly allocated to two or more groups (treatment and control) and the outcomes of the groups are compared after sufficient follow-up time.
A study that shows the efficacy of a diagnostic test is called a prospective, blind comparison to a gold standard study. This is a controlled trial that looks at patients with varying degrees of an illness and administers both diagnostic tests -- the test under investigation and the "gold standard" test -- to all of the patients in the study.
Cohort studies identify a large population who already has a specific exposure or treatment, follows them over time (prospective), and compares outcomes with another group that has not been affected by the exposure or treatment being studied. Cohort studies are observational and not as reliable as randomized controlled studies, since the two groups may differ in ways other than in the variable under study.
Case control studies are studies in which patients who already have a specific condition or outcome are compared with people who do not. Researchers look back in time (retrospective) to identify possible exposures. They often rely on medical records and patient recall for data collection. These types of studies are often less reliable than randomized controlled trials and cohort studies because showing a statistical relationship does not mean than one factor necessarily caused the other.
Case series and Case reports consist of collections of reports on the treatment of individual patients or a report on a single patient. Because they are reports of cases and use no control groups with which to compare outcomes, they have no statistical validity.
In order be successful in using Evidence Based Practice (EBP) you will need to learn how to develop well-composed clinical questions. By formatting your research question in a PICO(T) format you can gather evidence relevant to your patient's problem. Well-composed PICO(T) questions generally contain up to four components each represented in the acronym "PICO(T)" P=Patient or Population and Problem; I=Intervention or Indicator; C=Comparison or Control (not part of all questions); O=Outcome; T=Time or Type.
To formulate questions in Evidence Based Practice, use the PICO format. 
PICO(T) stands for:
Your PICO question will fall under one of these types:
Note: Image from California State University, Long Beach Library