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## Experimental / exposed group

Number of cases that experienced an adverse event or bad outcome :   (a)

Number of cases that did NOT experience an adverse event or bad outcome :  (b)

## Control group

Number of cases that experienced an adverse event or bad outcome :   (c)

Number of cases that did NOT experience an adverse event or bad outcome :  (d)

#### Background Info

Odds Ratio
> Ratio of the odds of an event occurring in one group to the odds of it occurring in another group.  The groups can be any dichotomous classification such as Male/Female, Experimental/Control, Exposed/non-exposed, etc.

### Outcome status

Positive Negative

### Exposure status

Positive (experimental) a b
Negative (control) c d

Odds Ratio = [ a /c] /  [b / d]   =  [a * d] / [b * c]
Note: if a zero is entered in any of the boxes, the odds ratio will be estimated using the null hypothesis.

>Used in Case-control studies - estimate of the relative incidence (relative risk) of the outcome associated with exposure.

If the odds ratio is equal to one, exposure does not affect the odds of outcome.  If the odds ratio is less than one, exposure is assocated with a lower odds of outcome and if greater than one,  exposure is assocated with a higher odds  od outcome .

Confidence interval = exp[ ln(OR) +/-  z * SE{ln(OR} ]
z (standard normal deviate):  90% CI  (z= 1.645).   95% CI (z= 1.96).   99% CI (z= 2.576).

SE{ln(OR}  = sqrt [ 1/a + 1/c + 1/b + 1/d ]

Large confidence intervals represent a low level of precision of the OR, while a small confidence interval represents a higher level of precision.

## References

1] Zhang J, Yu KF. What’s the relative risk? A method of correcting the odds ratio in cohort studies of common outcomes. JAMA 1998;280:1690- 1691.

2] McNutt LA, Wu C, Xue X, Hafner JP. Estimating the relative risk in cohort studies and clinical trials of common outcomes. Am J Epidemiol 2003;157:940-943.

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