Types of studies we are going to cover all fall under one of two categories - primary sources or secondary sources. Primary sources are those that report original research and secondary sources are those that compile and evaluate original studies.
Randomized Controlled Trials are studies in which subjects are randomly assigned to two or more groups; one group receives a particular treatment while the other receives an alternative treatment (or placebo). Patients and investigators are "blinded", that is, they do not know which patient has received which treatment. This is done in order to reduce bias.
Cohort Studies are cause-and-effect observational studies in which two or more populations are compared, often over time. These studies are not randomized.
Case Control Studies study a population of patients with a particular condition and compare it with a population that does not have the condition. It looks the exposures that those with the condition might have had that those in the other group did not.
Cross-Sectional Studies look at diseases and other factors at a particular point in time, instead of longitudinally. These are studies are descriptive only, not relational or causal. A particular type of cross-sectional study, called a Prospective, Blind Comparison to a Gold Standard, is a controlled trial that allows a research to compare a new test to the "gold standard" test to determine whether or not the new test will be useful.
Case Studies are usually single patient cases.
Systematic Reviews are studies in which the authors ask a specific clinical question, perform a comprehensive literature search, eliminate poorly done studies, and attempt to make practice recommendations based on the well-done studies.
Meta-Analyses are systematic reviews that combine the results of select studies into a single statistical analysis of the results.
Practice Guidelines are systematically developed statements used to assist practitioners and patients in making healthcare decisions.
Different types of clinical questions have certain kinds of studies that best answer them. The chart below lists the categories of clinical questions and the studies you should look for to answer them.
Evidence-Based Medicine is the integration of best research evidence with clinical expertise and patient values. (Sackett DL, Straus SE, Richardson WS, et al. Evidence-based medicine: how to practice and teach EBM. 2nd ed. Edinburgh: Churchill Livingstone, 2000.)
Unfiltered resources are primary sources that describe original research. Randomized controlled trials, cohort studies, case-controlled studies, and case series/reports are considered unfiltered information.
Filtered resources are secondary sources that summarize and analyze the available evidence. They evaluate the quality of individual studies and often provide recommendations for practice. Systematic reviews, critically-appraised topics, and critically-appraised individual articles are considered filtered information.
For more in-depth help with finding unfiltered and/or filtered sources, check out this guide.