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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.
Primary Sources
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.
Secondary Sources
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.
PICO is a useful way of formulating clinical research questions and a well-build question or problem should include the four components of the model: Patient/Problem, Intervention, Comparison and Outcome.
PICO Linguist is a tool from the NLM that lets you search for research with terms that incorporate the PICO model. Using it, you can limit your search results to certain types of studies (clinical trials, randomized controlled trials, etc.) or to practice guidelines.
| Patient/Problem | Describe the patient. Important descriptors might include: age and gender. Then describe the problem the patient is experiencing. For example, you might say, "A four-year-old boy with asthma" |
| Intervention | Describe the treatment you are considering for the patient. This may be a drug, such as "theophylline" for the child with asthma |
| Comparison | Ask yourself what main alternative therapy exists for the problem. Example: "inhaled glucocorticosteroids" |
| Outcome | Ask yourself what result you want to see because of the therapy. Example: "decreased hospitalizations and school abscences" |
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.
