Box 9.2.a: Calculation of risk ratio (RR), odds ratio (OR) and risk difference (RD) from a 2×2 table

The results of a clinical trial can be displayed as a 2×2 table:

 

Event

(‘Success’)

No event

(‘Fail’)

Total

Experimental intervention

SE

FE

NE

Control intervention

SC

FC

NC

where SE, SC, FE and FC are the numbers of participants with each outcome (‘S’ or ‘F’) in each group (‘E’ or ‘C’). The following summary statistics can be calculated: