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EBM Resources

Study Designs for PICO

Knowing the type of foreground question can help you select the best study design to answer your question. You always want to look for the study design that will yield the highest level of evidence. Remember, foreground questions can be further divided into questions that relate to therapy, diagnosis, prognosis, etiology/harm.

Below is a different type of pyramid of that ranks studies by the strength of the evidence they provide. Experimental designs are okay for therapy and diagnosis questions, but you cannot experiment on your subjects in the cases of etiology or harm. (You cannot deliberately expose them to something harmful). In questions of harm or etiology, you can only observe what has happened (retrospective cohort study or case-control) or what will likely will happen to people (prospective cohort study). This only works if that type of data has been/will be collected.

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Definitions of Study Types
(From BMJ’s Clinical Evidence Glossary)

Meta-analysis: A statistical technique that summarizes the results of several studies in a single weighted estimate, in which more weight is given to results of studies with more events and sometimes to studies of higher quality.

Systematic Review: a review in which specified and appropriate methods have been used to identify, appraise, and summarize studies addressing a defined question. (It can, but need not, involve meta-analysis). In Clinical Evidence, the term systematic review refers to a systematic review of RCTs unless specified otherwise.

Randomized Controlled Trial: a trial in which participants are randomly assigned to two or more groups: at least one (the experimental group) receiving an intervention that is being tested and another (the comparison or control group) receiving an alternative treatment or placebo. This design allows assessment of the relative effects of interventions.

Controlled Clinical Trial: a trial in which participants are assigned to two or more different treatment groups. In Clinical Evidence, we use the term to refer to controlled trials in which treatment is assigned by a method other than random allocation. When the method of allocation is by random selection, the study is referred to as a randomized controlled trial (RCT). Non-randomized controlled trials are more likely to suffer from bias than RCTs.

Cohort Study: a non-experimental study design that follows a group of people (a cohort), and then looks at how events differ among people within the group. A study that examines a cohort, which differs in respect to exposure to some suspected risk factor (e.g. smoking), is useful for trying to ascertain whether exposure is likely to cause specified events (e.g. lung cancer). Prospective cohort studies (which track participants forward in time) are more reliable than retrospective cohort studies.

Case Control Study: a study design that examines a group of people who have experienced an event (usually an adverse event) and a group of people who have not experienced the same event, and looks at how exposure to suspect (usually noxious) agents differed between the two groups. This type of study design is most useful for trying to ascertain the cause of rare events, such as rare cancers.

Case Series: analysis of series of people with the disease (there is no comparison group in case series).

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