Relative Risk (RR) and Odds Ratio (OR) measure the strength of an association between an exposure and a disease.
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Relative Risk (RR)
- Tells you how many times exposure to a risk factor increases a person’s risk of contracting the disease.
- Is the ratio of incidence of disease in exposed persons to the incidence of disease in unexposed persons.
- Studies that create a sample population based on exposure can use relative risk- like controlled trials or cohort studies.
- Formula: RR = p of getting the disease if exposed / p of getting the disease if not exposed
Interpretation
- RR = 1
- If the incidence in the exposed group and the unexposed group is the same then there is no increased risk and no association between the exposure and the outcome.
- RR > 1
- The incidence in the exposed group is greater than that of the unexposed group, so there is a greater risk and a positive association between the exposure and the outcome.
- RR < 1 The
- incidence in the exposed group is less than that of the unexposed group so there is decreased risk and a negative association between the exposure and the outcome.
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Odds Ratio (OR)
- A clinical odds ratio usually refers to the odds that a person with a disease was exposed to a risk factor in the past divided by the odds that the control group had exposure to the risk factor.
- Is a way of comparing patients that already have a certain condition with patients that don’t.
- Case Control studies compare cases that have experienced the event and controls who haven’t and then looks at whether each individual was exposed or not.
- Formula: OR = odds of the exposure in people with disease / odds of the exposure in people without disease.
Interpretation
- OR = 1
- The exposure is not associated with the disease.
- OR > 1
- The exposure is positively associated with the disease.
- OR < 1 The
- The exposure is negatively associated with the disease.
Since all the cases already have the disease/outcome, you can’t calculate incidence and therefore can’t calculate risk.
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RR is calculated using incidence data to measure the probability of developing disease. Since the calculation is dependent on incidence data and only Controlled Trials and Cohort Studies can be used to calculate incidence, you should only see RR in Controlled Trials or Cohort Studies. This is also why you see RR less than OR even though it is easier to calculate and understand – Controlled Trials and Cohort Studies are expensive and time consuming so there are fewer of them.
