060920L - MEASURE OF EXCESS DISEASE RISK: THE ODDS RATIO AND THE RATE (RISK) RATIO

Lecture for Year 2 Semester 1 medical student PPSD session on 20^th September 2006 by Professor Omar Hasan Kasule Sr.

Excess disease risk is measured as an absolute effect (Rate Difference or Risk Difference) or a relative effect (Odds Ratio and Rate Ratio). Other terms with meaning similar to rate ratio are relative risk and risk ratio  

The following 2x2 contingency table shows the lay-out of data that can be used to define OR and RR.

	Disease +	Disease -		time
Exposure +	a	B	a + b	T+
Exposure -	c	D	c + d	T-
	a + c	b + d	N	T

The Odds ratio is as OR = ad/bc by reference to the 2 x 2 contingency table above.

The Rate Ratio is defined as RR = (a/T+) / (a/T-)

A ratio of 1.0 is called the null value and is interpreted to mean that there is no relation between the disease and the exposure.

A ratio above 1.0 means that the exposure increases the risk of disease

A ratio below 1.0 means that the exposure protects from the disease.

The OR and RR are often not very different numerically. The OR is used for case control studies and the RR is used for follow-up studies.