Odds ratios (Basic concepts)

Odds Ratio: Definition, Calculation, Interpretation.

1. Odds Ratio Odds Ratio Basic Concepts Odds Ratio Dr. Alazabee Tarek Group Discussion

2. ODDS RATIO

3. 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). Ratio = a / b تناسب

4. ODDS : The probability that an event will occur divided by the probability that the event will not occur. ODDS = P / (1-P) رجحان ، تحيز

5. Odds Ratio ( OR ) Definition: Is the ODDS that a patient is exposed to the risk factor divided by the ODDS that a control is exposed

6. Table .: Data for odds ratio for stroke with history of drug abuse Stroke Control Drug Abuse 73 18 No Drug Abuse 141 196 Total 214 214

7. Calculation

8. STEP One : Calculate the odds that a patient ( With stroke) is exposed ( Abused drugs ) : 1 1 2 2 Exposed Not Exposed Cases The probability that an event will occur The probability that the event will not occur.

9. STEP Two : Calculate the odds that a control ( Without stroke) is exposed ( Abused drugs ) : 1 1 2 2 Exposed Not Exposed The probability that an event will occur The probability that the event will not occur.

10. STEP Three ODD ODD

11. Interpretation the odds is 5 .64 That is, a person in this study who abuses drugs is more than five times more likely to have a stroke

12. An odds ratio of

13. SO,, An odds ratio is measure of association which quantifies the relationship between an exposure and health outcome from a comparative study. This feature is particularly relevant when we analyze data from a case-control study , which has a group of cases and a group of non-cases or controls.