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Evidence-Based Dentistry: Determining
Strength of Evidence


  • Bias refers to the systematic error that occurs in the design, conduct or interpretation of a study which may cause a deviation from the underlying truth.
  • Confounders occur when a factor disturbs the true relationship of the study variable because it is also relating to the outcome variable being measured.

Study Types- Definitions

  • Meta-analysis: A statistical analysis that combines or integrates the results of several independent clinical trials considered by the analyst to be "combinable" usually to the level of re-analyzing the original data; also sometimes called pooling or quantitative synthesis.
  • Systematic review: A review of a body of data that uses explicit methods to locate primary studies, and explicit criteria to assess their quality.
  • Randomized Controlled Trials: Individuals are randomly allocated to a control group and a group who receive a specific intervention. Otherwise the two groups are identical for any significant variables. They are followed up for specific end points.
  • Cohort Studies: Groups of people are selected on the basis of their exposure to a particular agent and followed up for specific outcomes.
  • Case Control Studies: Patients ("cases") with the condition are matched with "controls" without, and a retrospective analysis is used to look for differences between the two groups.
  • Case Study: A report based on a single patient or subject; sometimes collected together into a short series.
  • Expert Opinion and Anecdotal: A consensus of experience from the good and the great. Ex: Something someone told you after a meeting.

Types of Question

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


Evidence Pyramid

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." [1]

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." [2]



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

2. "confounding factor." In A Dictionary of Nursing, edited by Martin, Elizabeth A., and Tanya A. McFerran. : Oxford University Press, 2014. 

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