Primary Literature (Unfiltered Literature)
Case series / Case reports: reports on the treatment of one or more individual patients. There are no control groups to compare outcomes. This type of research has little statistical validity.
Case control studies: patients who already have a specific condition are compared with people who do not have the condition, looking back in time to identify factors that might be associated with the illness. Case control studies often rely on medical records & patient recall for data collection, so their reliability is limited.
Cohort studies: compare a group of patients who are already under a specific treatment with a similar group not affected by the treatment. Both groups are are followed over time. Cohort studies are observational, so their reliability is limited.
Randomized controlled clinical trials (RCTs): carefully planned experiments that look at the effects of a treatment on real patients in real time. To reduce the potential for bias, randomization & blinding are used in the selection of patients & treatments. Patients are assigned to treatment & control groups randomly; patients & clinicians (& sometimes laboratories) do not know who is in which group & thus who is receiving treatment or not. This allows the groups to be directly compared. RCTs can provide sound evidence of cause & effect & are considered the gold standard for experimental research.
Secondary Literature (Filtered Literature)
Systematic Reviews: answer a specific question, using the results of an extensive literature search that identifies studies with sound & similar methodologies. The studies are reviewed, assessed for quality, & the results are summarized according to the standards of the question that is being reviewed.
Meta-analyses: thoroughly examine a number of valid studies on a topic & mathematically combine the results using statistical methodology to report the results of the analysis as if it were one large study.