odds ratios (basic concepts)

Odds Ratio

Odds RatioBasic Concepts

Odds Ratio

Dr. Alazabee Tarek

Group Discussion

ODDS RATIO

Definition

Calculation

Interpretation

ODDS

Ratio

ODDS RATIO

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

ODDS

Ratio

ODDS RATIO

ODDS: The probability that an event will occur divided by the probability that the event will not occur.

ODDS = P / (1-P)

، رجحانتحيز

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

Case Control Study

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

Calculation

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.

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.

STEP Three

ODD

ODD

Interpretationthe 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

An odds ratio of

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.