Table 1

Impact upon power and required sample size due to dichotomisation

Power to detect association (%)IOP as a continuous variable noIOP as a binary variable
Cutpoint (mm Hg)ndPower if n=no* (%)
9011914.517573
1620767
1321267
80 9014.513361
1616155
1316254
• 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=no (as for IOP as a continuous variable) and IOP is dichotomised and analysed accordingly.

• IOP, intraocular pressure; SBP, systolic blood pressure.