What is it with odds and risk?!
Posted on 8th May 2013 by Geethu Parvathy
When it comes to analytical studies like cohort studies and case control studies, these two words odds ratio and relative risk, may get confused with each other sometimes, especially for beginners. Both are just two distinct terms used to denote “strength of association”.
Strength of association:
Refers to how much a particular factor is associated directly with the disease/condition under study. For example “smokers have 6 times increased risk of getting lung cancer than non-smokers” here the value “6” denotes the strength of causal association between smoking and lung cancer.
What is odds ratio?
Odds ratio is a measure of strength of association between risk factor and outcome/disease in a case control study. 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.
Here the outcome being investigated should not be a common one, the cases must represent those who already had the outcome and controls represent those without outcome.
, , Case, Control
Risk Factor, Present, a, b
, Absent, c, d
Odds ratio is only a rough method of estimating the strength of association because a case control study never provide incidence rates since there is no definite population under risk. Odds ratio = ad/bc
What is relative risk?
It is again a measure of strength of association. Relative risk is actually the ratio between incidence of outcome/disease among exposed people and that among unexposed people. It is usually used in a cohort study where there is a definite population under study and we can calculate incidence rates. Hence it is a direct and accurate value compared to odds ratio. Here also one can create a 2*2 table.