1. 17
Risk Ratio
The relative risk (RR) is the probability that a member of an
exposed group will develop a disease relative to the
probability that a member of an unexposed group will
develop that same disease.
/ exp
= / exp
disease osed
RR disease not osed
RR =1: association between exposure and disease unlikely
to exist.
RR= >1 increased risk of disease among those that have
been exposed.
RR = < 1decreased risk of disease among those that have
been exposed.
How far does RR need to be from 1 so that we can say with
some confidence that exposure have some effect on disease?
Example:
Disease is breast cancer (BC). A woman is considered to be
exposed if she gave birth (a baby) at or after the age of 25.
Exposure factor Disease or
condition
Total
Breast
ca
positive
Breast ca
negative
Exposed (gave
birth before 25)
2(a) 103 (b) 105 (a+b)
Not Exposed (do
not gave birth
before 25)
3 (c) 207 (d) 210 (c+d)
total 5 310 315
2. Biostatistics-151
/ exp
= / exp
disease osed
RR disease not osed
/( )
= /( )
a a b
RR c c d
+
+
2/105
= =1.33
3/ 210
RR
The collection of women who gave birth at a later age (>= 25)
are at increased risk for developing Breast ca.
Risk ratio
[Q:
Write short notes on: Risk ratio. (BSMMU, July 2010,
July 2011)
How the risk factors are assessed? Mention the
appropriate use of odds ratio and risk ratio. [BSMMU,
January, 2012
How will you quantify risk factors for cohort and case-
control study? (BSMMU, MD Radiology, July, 2010)
Odds Ratio
The odds ratio (OR) is the odds of disease among exposed
individuals divided by the odds of disease among unexposed.
In above example
Odd of exposed= a/b
Odd of unexposed= c/d
/
,odds ratio (OR) = /
a b
So c d
Example:
From the data of above example, we can calculate OR as
follows:
2/103
odds ratio (OR) = =1.34
3/ 207
[Q. In a cohort study 250 women of urban area & 150 female of
rural area were followed up 12 urban women & 18 rural women