Lead Author(s): Jeff Martin, MD

Definition of Association

Measuring Disease Association

Usually we want to look at the relationship between an exposure (risk factor, predictor) and the outcome.

The measures that are used to show association beween an exposure or treatment and disease are called measures of association.

Summary of Measures of Association

The following table shows all the possible ratio and difference measures that can be calculated from cross-sectional, cohort data, and case control study.

Study Type Ratio Difference
Cross-sectional Study prevalence ratio prevalence difference
Cross-sectional Study odds ratio odds difference
Cohort Study risk ratio risk difference
Cohort Study rate ratio rate difference
Cohort Study odds ratio odds difference
Case Control Study case control odds ratio  

All of the ratios are commonly seen, but the difference measures are rare except for the risk difference and possibly the rate difference.

Ratio Measures Limited by Type of Study

The type of study limits the kind of ratio measure that can be calculated.

Relative Measures and Strength of Association

In practice many risk factors have a relative measure in the range of 2 to 5: Some very strong risk factors may have a relative measure in the range of 10 or more Relative measures < 2.0 may still be valid but are more likely to be the result of bias These ranges for ratio measures can be used as a kind of rough rule of thumb. There is certainly nothing absolute about them.

They suggest that it is more difficult to demonstrate an etiologic role for exposures that have associated ratios less than 2.0.