Table 1

Impact upon power and required sample size due to dichotomisation

Power to detect association (%)	IOP as a continuous variable n_o	IOP as a binary variable
Power to detect association (%)	IOP as a continuous variable n_o	Cutpoint (mm Hg)	n_d	Power if n=n_o* (%)
90	119	14.5	175	73
		16	207	67
		13	212	67
80	90	14.5	133	61
		16	161	55
		13	162	54

Assumptions of the model: both IOP and SBP follow a normal distribution with means equal to 14.5 and 135 mm Hg, respectively, and SDs equal to 2.4 and 20 mm Hg, respectively.
*Statistical power if sample size=n_o (as for IOP as a continuous variable) and IOP is dichotomised and analysed accordingly.
IOP, intraocular pressure; SBP, systolic blood pressure.