Evidence-Based Practice

Systematic Reviews and Meta-Analyses are generally considered to be the highest quality of evidence. But the type of study you need may differ based on what type of research question you are trying to answer. You'll want to identify which are the most valid research designs that can "prove" the research question with the understanding that there may not be much research done on that research question.

Table source: OHSU's Evidence-Based Practice Toolkit for Nursing

"Evidence Pyramid" is a product of Tufts University and is licensed under BY-NC-SA license 4.0

Systematic Review 
A summary of the clinical literature. A systematic review is a critical assessment and evaluation of all research studies that address a particular clinical issue. The researchers use an organized method of locating, assembling, and evaluating a body of literature on a particular topic using a set of specific criteria. A systematic review typically includes a description of the findings of the collection of research studies. (AHRQ Glossary of Terms)

Meta-Analysis 
A work consisting of studies using a quantitative method of combining the results of independent studies (usually drawn from the published literature) and synthesizing summaries and conclusions which may be used to evaluate therapeutic effectiveness, plan new studies, etc. It is often an overview of clinical trials. It is usually called a meta-analysis by the author or sponsoring body and should be differentiated from reviews of literature. (PubMed)

Randomized Controlled Trial 
A controlled clinical trial that randomly (by chance) assigns participants to two or more groups. There are various methods to randomize study participants to their groups. (AHRQ Glossary of Terms)

Controlled Clinical Trial 
A type of clinical trial comparing the effectiveness of one medication or treatment with the effectiveness of another medication or treatment. In many controlled trials, the other treatment is a placebo (inactive substance) and is considered the "control." (AHRQ Glossary of Terms)

Cohort Study 
A clinical research study in which people who presently have a certain condition or receive a particular treatment are followed over time and compared with another group of people who are not affected by the condition. (AHRQ Glossary of Terms)

Case Control Study
The observational epidemiologic study of persons with the disease (or other outcome variable) of interest and a suitable control (comparison, reference) group of persons without the disease. The relationship of an attribute to the disease is examined by comparing the diseased and nondiseased with regard to how frequently the attribute is present or, if quantitative, the levels of the attribute, in each of the groups. (OCEBM Table of Evidence Glossary)

Case Series
A group or series of case reports involving patients who were given similar treatment. Reports of case series usually contain detailed information about the individual patients. This includes demographic information (for example, age, gender, ethnic origin) and information on diagnosis, treatment, response to treatment, and follow-up after treatment. (OCEBM Table of Evidence Glossary)

Case Study
An investigation of a single subject or a single unit, which could be a small number of individuals who seem to be representative of a larger group or very different from it. (Dictionary of Nursing Theory and Research, Fourth Edition)

Editorial
Work consisting of a statement of the opinions, beliefs, and policy of the editor or publisher of a journal, usually on current matters of medical or scientific significance to the medical community or society at large. The editorials published by editors of journals representing the official organ of a society or organization are generally substantive. (PubMed)

Opinion
A belief or conclusion held with confidence but not substantiated by positive knowledge or proof. (The Free Dictionary)

Animal Research
A laboratory experiment using animals to study the development and progression of diseases. Animal studies also test how safe and effective new treatments are before they are tested in people.(NCI Dictionary of Cancer Terms)

In Vitro Research
In the laboratory (outside the body). The opposite of in vivo (in the body). (NCI Dictionary of Cancer Terms)