Strength of the Evidence: Interpretation of OR and RR

It is interesting to note that in the literature, particularly in narrative/literature review articles, when the author is talking about an association between periodontal disease and other systemic conditions, the term odds ratio (OR) is more frequently referred to than the term relative risk (RR). If one recalls from the definitions of these terms listed in the glossary, the definition for OR states that it is a “measurement of association” used in Case-Control Studies. Taking a look at the Evidence Pyramid illustrated in Figure 2, it is noted that Case-Control Studies are Level 3 Evidence and not typically used to determine causality. Only RCT’s or Cohort Studies (or Systematic reviews of these 2 types of studies) can be used to determine causality in order to fulfill two of the Bradford Hill criteria: “Strength of the Evidence” and “Experiment.” Since RR is the appropriate “measurement of association” for Cohort studies and RCT’s, one must look for references to those measurements when reading such studies. Thus, it is important to understand that when OR’s are cited, they cannot be as seriously applied to a causal relationship, as can RR’s. Additionally, Sackett et al.8 suggested when referring to Strength of the Evidence, if considering OR’s, then only OR’s greater than 4.0 should be considered as “strong”, particularly if the study was a case-control study. Some cohort studies use OR’s rather than RR’s, thus consideration of the study type can also influence the strength of the association (i.e., OR’s >3.0 may be considered strong in cohort studies). He also indicated that although a relationship may be statistically significant, the strength of the association may still be weak (e.g., OR <2.0).