Mercury sphygmomanometers	Aneroid sphygmomanometers
BHS standard not achieved	4	41
BHS standard achieved	71	150
Total	75	191

Relative risk = (4/75)/(41/191) = 0.25; odds ratio = (4/71)/(41/150) = 0.21

Because the outcome of interest (devices failing) is ‘rare’, the risk of failure (number of devices failing divided by number of all devices) is close in value to the odds of failure (number of devices failing divided by number of devices not failing); the RR and OR will, therefore, be close in value. In epidemiology, this is called the ‘rare disease assumption’.

BHS = British Hypertension Society