The evidence pyramid is used to illustrate the evolution of the literature.
As you move up the pyramid the amount of available literature decreases, but increases in its relevance to the clinical setting.
You will encounter many types of evidence, it is important to distinguish between these different categories of scholarly literature.
Peer-reviewed (or refereed): Refers to articles that have undergone a rigorous review process, often including revisions to the original manuscript, by peers in their discipline, before publication in a scholarly journal. This can include empirical studies, review articles, meta-analysis among others.
Empirical study (or primary article): An empirical study is one that aims to gain new knowledge on a topic through direct or indirect observation and research. These include quantitative or qualitative data and analysis. In science, an empirical article will often include the following sections: Introduction, Methods, Results, and Discussion.
Review article: In the scientific literature, this is a type of article that provides a synthesis of existing research on a particular topic. These are useful when you want to get an idea of a body of research that you are not yet familiar with. It differs from a systematic review in that it does not aim to capture ALL of the research on a particular topic.
Systematic review: This is a methodical and thorough literature review focused on a particular research question. It's aim is to identify and synthesize all of the scholarly research on a particular topic in an unbiased, reproducible way to provide evidence for practice and policy-making. It may involve a meta-analysis (see below).
Meta-analysis: This is a type of research study that combines or contrasts data from different independent studies in a new analysis in order to strengthen the understanding of a particular topic. There are many methods, some complex, applied to performing this type of analysis.
Filtered or secondary resources are summaries and analysis of the evidence derived from and based on unfiltered ( primary sources) . They provide an appraisal of the quality of studies and often make recommendations for practice.
What kinds of studies are relevant?
Here are some methods to tell if a journal is peer-reviewed:
The Journal Citations Reports (JCR) provides quantitative tools for ranking, evaluating, categorizing, and comparing journals. One measurement used is the Impact Factor. The Impact Factor is a measure of the frequency with which the "average article" in a journal has been cited in a particular year or period. The Impact Factor is calculated by dividing the number of current year citations to the source items published in that journal during the previous two years. The Impact Factor should be used with caution as there are many factors that can influence citation rates.
To learn more about Impact Factor read this article: The Agony and the Ecstasy—The History and Meaning of the Journal Impact Factor