Question

Describe the relationship between risk ratio and odds ratio. In what ways do these concepts help guide epidemiological research? In what study design(s) would you most likely use these ratios?

Answer 1

Ans) Odds Ratio: It is defined as the ratio of the odds of an event occurring in one group to the odds of it occurring in another group or to a sample-based estimate of that ratio= [A/(1-A)]/[B/(1-B)].

Relative risk (RR) is the risk of an event (or of developing a disease) relative to exposure=A/B.

Relationship between risk ratio & odds ratio:

- In general conversation the terms ‘risk’ and ‘odds’ are used interchangeably as if they describe the same quantity.

- In statistics, however, risk and odds have particular meanings and are calculated in different ways.

- When the difference between them is ignored, the results of a systematic review may be misinterpreted.

Applying Concept of these in Epidemiological research:

- So, the odds of having the disease is the ratio of the probability that the disease will occur to the probability that the disease will not occur.

Or, the odds of having the disease can be calculated as the number of people with the disease divided by the number of people without the disease

[Note: in the exposure-disease 2x2 table, the odds of having a disease in the exposed group is the same as the odds that an exposed person develops the disease]

- Odds ratio is the ratio of the odds of disease in the exposed to the odds of disease in the non-exposed

OR = odds that an exposed person develops the disease / odds that a non - exposed person develops the disease

Odds ratio can be calculated in a cohort study and in a case control study

− The exposure odds ratio is equal to the disease odds ratio

- Relative risk can only be calculated in a cohort study

- When the “cases” studied are representative of all people with the disease in the population from which the cases were drawn, with regards to history of the exposure

- When the “controls” studied are representative of all people without the disease in the population from which the cases were drawn, with regards to history of exposure

- When the disease being studied is not a frequent one