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

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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
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