Types of studies fall under one of two categories: primary and secondary sources. Primary sources are those that report original research. Secondary sources are those that compile and evaluate original studies.
Randomized Controlled Trials (RCT) are studies in which subjects are randomly assigned to treatment/intervention group or a control/placebo group. Patients and investigators are blinded, that is they do not know which patients has received which treatment. Randomization and blinding are done in order to reduce bias.
Best design for studying the effect of an intervention.
Cohort Studies are cause-and-effect observational studies in which two or more populations are compared, often over time. These studies are not randomized.
Used to study the effect of predictive risk factors of an outcome.
Case-Control Studies look at a population of patients with a particular condition and compare it with a population that does not have the condition. It looks at the exposures that those with the condition might have had that those in the other group did not.
Cross-Sectional Studies look at the diseases and other factors at a particular point in time, instead of longitudinally. These studies are descriptional only, not relational or causal.
Useful for quantifying the prevalence of a disease or risk factor, and for quantifying the accuracy of a diagnostic test.
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 conducted studies and attempt to confirm current practice or highlight the need for further research.
Meta-Analyses are reviews that combine the results of studies into a single statistical analysis of results. Authors of systematic reviews often pool results using meta-analysis.
Practice Guidelines are systematically developed statements used to assist practitioners and patients in making healthcare decisions.
The strength of the evidence produced varies among the different types of studies. Filtered sources like systematic reviews and meta-analyses provide stronger evidence because they evaluate and compare a number of original studies. The image below demonstrates the relative strengths of the study types - generally, the higher up on the pyramid you go, the more rigorous the study design and the lesser likelihood of bias or systematic error.