Odd’s Ratio
DEFINITION:
- ODDS:The probability that an event will occur divided by the probability that the event will not occur
- Odds = Probability/(1-probability)
RATIO:
- Is the number (a) of observations in a given group with a given characteristic ( such as having an MI) divided by the number (b) of observations without the given characteristic (such as not having MI).
ODDS RATIO:
- Is the ODDS that a patient is exposed to the risk factor divided by the ODDS that a control is exposed
Odd ratio in case control study:
|
Case |
control |
|
|
Exposed |
a |
b |
|
unexposed |
c |
d |
- Odd of exposure among the cases: a/c
- Odd of exposure among the control: b/d
- Therefore Exposure odd ratio is: a/c ⁄b/d
a/c = ad
b/d bc
- In a case control study, the most characteristic feature is odd’s ratio
Odds ratio in cohort study:
|
Outcome YES |
Outcome No |
|
|
Exposed |
a |
b |
|
unexposed |
c |
d |
- Odd of outcome among exposed=a/b
- Odd of outcome among unexposed=c/d
Odd ratio= a/b = ad/bc
c/d
- 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.
- Odds ratio can be a measure of relative risk in case control study.
Odds ratio is a measure of true relative risk when:
- The outcome is relatively rare. ( prevalence is 10% or less in general population)
- The cases must be representative of the cases in the population.
- The controls must be representative of the controls in the population
Interpretation of odds ratio (OR) :
- OR of >1 indicates that the exposure is associated with an increased risk of developing the disease.
- OR of Closer the value of OR to 0 greater the protection.
- The OR will rarely equal to 1 in absence of true risk or protection
Uses of OR
- OR are appropriate measure of RR in case control studies.
- OR are commonly used in meta-analysis.
- Odds ratio is the measure of the strength of the association between risk factor and outcome.
- OR are the output of logistic regression analysis.
Disadvantage:
- OR exaggerate RR in common outcomes(prevalence>10).
- In those cases, if OR >1 it is larger than the true RR and conversely if OR
Exam Question
- In a study done to establish Smoking as a risk factor for a disease, 30 out of 50 smokers developed the disease while 10 out of 50 non-smokers developed the disease. Odd’s ratio is 6
- Odds ratio is related to Relative risk
- Odds = Probability/(1-probability)
- Using case control study only odds ratio can be calculated.
- Association can be measured by odds ratio
- In a case control study, the most characteristic feature is odd’s ratio
- Odds ration is designated as ad/bc
Don’t Forget to Solve all the previous Year Question asked on Odd’s Ratio
