In a group of cancer pts, 10 died that wasn’t treated while 5 died in the tx group. Which
statement is correct?
a. Absolute risk =10
b. Relative risk =10
c. Relative risk =5
d. Absolute risk=5
e. Relative risk=2
answer: D
because this is the incidence in the exposed
RR would be 5/10 =0.5
Absolute risk vs Relative risk explained
-
Relative risk : probability of an event happening
calculated by:
incidence in exposed group/incidence in non exposed -
Relative risk reduction:
calculated by: (1 - Relative risk) x 100
-
Absolute risk: incidence in an exposed group
-
Absolute risk reduction(used if there was a favorable outcome):
control rate - experimental rate -
Absolute risk increase(used if there was an undesirable outcome):
experimental rate - control rate -
Number needed to treat: calculated by
1 / ARR
For example:
1000 patients given a placebo, 20 died
1000 patients given a drug, 10 died
Relative risk= 10/20 = 0.5 … meaning half as likely to die
Relative Risk Reduction = 1 - 0.5 x 100 = 50 % less likely to die
Absolute risk = 10/1000 for drug, 20/1000 for no treatment
Absolute risk reduction=0.02 - 0.01 = 0.01
Number needed to treat = 1/0.01 = 100