Not all levels of evidence are created equal. Depending on the type of question you ask, some study types will be more appropriate than others to use as evidence.
Cantrell, S., von Isenburg, M. Evidence-based practice for the medical librarian [Course content]. Medical Library Association & University of North Carolina at Chapel Hill School of Information and Library Science, Sakai. http://www.medlib-ed.org/products/1123/evidence-based-medicine-for-the-medical-librarian
for availability of full-text
In clinical research, the quality of your evidence is important. The stronger your research evidence is, the less potential for bias exists.
However, you also have to consider the probability of a topic having an abundance of articles in some of the categories.
For example, a new drug therapy for a rare disease that came out within the last year will not have many randomized controlled trials (RCTs) yet. Perhaps the trials are still ongoing. Without enough RCTs, there cannot be a systematic review written.
Authors systematically identify, select, evaluate, and synthesize all high-quality primary research in order to answer a focused clinical question. A meta-analysis uses statistical methods to combine data from studies included in a systematic review.
Look for words like 'systematic review' or 'meta-analysis' and a methods section that details the search strategy and inclusion criteria used to locate research included in the study.
Guidelines provide a summary of the relevant medical literature. Professional societies develop guidelines to aid clinicians in choosing diagnostic tests, treatments for specific conditions, etc.
Look for 10 or more authors and 'practice guideline' in the title of the article.
A scientific experiment where patients are divided into two groups. One group receives treatment and the other receives a placebo.
Look for a methods section in the article's headings and words like 'randomized' and 'placebo.'
An observational study where patients are followed over time to investigate causes of disease and to establish risk factors or health outcomes.
Look for 'cohort' or a group studied over a number of years and a section called 'methods.'
Studies that report in-depth on a patient or group of patients with the same, rare, disease.
Look for words like 'case report' or 'case study.'
No empirical evidence is provided. A group of experts provide recommendations based on experience and authoritative sources.
Look for words like 'review' or authors summarizing other articles. They will often not contain a methods section.
Researchers perform experiments on cell cultures, tissue, and animals to find treatments for human disease.
Look for words like 'rats,' 'mice,' and other animals or collected samples, with 'experiment' and a section heading called methods.
Narrative information about a real person or incident. A story or recounting of an event. Not a formal article.
The referee shirt icon 
to the left of the title means the journal does peer-review!
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