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Centre for Evidence-
Based Medicine

Calculation of Odds Ratio (OR) / Relative Risk (RR)

Calculation of odds ratio/relative risk for the use of trimethoprim-sulfamethoxazole prophylaxis in cirrhosis:

Adverse event occurs
(infectious complication)
Adverse event doesn't occur
(no infectious complication)
Totals
Exposed to the treatment
(experimental)

1

a

29

b

30

a + b

Not exposed to the treatment
(control)

9

c

21

d

30

c + d

Totals

a + c

10

b + d

50

a + b + c + d

60

CER = c/(c+d)
= 0.30

EER = a/(a+b)
= 0.033

Control event odds = c/d
= 0.43

Experimental event odds = a/b
= 0.034

Relative risk = EER/CER
= 0.11

Relative odds = odds ratio
= (a/b)/(c/d)
= ad/bc = 0.08