Challenges in interpreting study results: the conflict between appearance and reality.

BACKGROUND: Many studies investigating the relationship between periodontal disease and systemic diseases have been reported; the majority of these have been epidemiologic (or observational) studies. The purpose of this article is to help readers understand the strengths and limitations of epidemiology for the purpose of being better able to interpret these studies.
FINDINGS: Epidemiologic studies include retrospective case-control studies and prospective cohort studies. While these studies cannot prove causality, they can provide strong evidence for and show the strength of an association between a disease and putative causative factors. Randomized controlled trials (RCTs) are used to test therapeutic and preventive measures and can provide presumptive evidence of disease causation in certain circumstances. Each of these study types has limitations that can distort the study results and, therefore, should be considered in study design and analysis. CONCLUSIONS AND CLINICAL IMPLICATIONS: Epidemiologic studies conducted to date suggest an association between periodontal disease and a number of systemic diseases. However, the strength and nature of this association are not yet clear, because in some cases it might result from confounding by smoking or other variables. Additional well-designed observational studies and future RCTs should increase our understanding of the actual relationship between periodontal and systemic diseases.