The type of question asked helps determine the most appropriate study type to use.
|Most common type of questions:||Type of study:|
how to select and interpret diagnostic tests
|prospective, blind comparison to a gold standard or cross-sectional|
how to select treatments that do more good than harm and that are worth the efforts and costs of using them
|randomized controlled trial > cohort study|
how to estimate the patient’s likely clinical course over time (based on factors other than the intervention) and anticipate likely complications of disease
|cohort study > case control > case series|
how to identify causes for disease (including iatrogenic forms)
|cohort > case control > case series|
An evidence pyramid is a visual representation study designs organized by strength of evidence. Study designs and publications shown at the top of the pyramid are considered thought to have a higher level of evidence than designs or publication types in the lower levels of the pyramid.
Strength of evidence is based on research design. The most scientific, rigorous study designs are randomized controlled trials, systematic reviews, and meta-analysis. These types of studies are thought to provide stronger levels of evidence because they reduce, but do not eliminate, potential biases and confounders. Bias is " systematic error, or deviation from the truth, in results or inferences." 
Confounding factors have "an effect on the dependent variable (and hence the outcome) that cannot be distinguished from the effect of the independent variable. This may lead to erroneous conclusions being drawn from the results of the experiment." 
1. Higgins JPT, Green S (editors). Cochrane Handbook for Systematic Reviews of Interventions Version 5.1.0 [updated March 2011]. The Cochrane Collaboration, 2011. Available from www.handbook.cochrane.org.
2. "confounding factor." In A Dictionary of Nursing, edited by Martin, Elizabeth A., and Tanya A. McFerran. : Oxford University Press, 2014.