A mean root caries increment of around 0.8 root surface per year of control group was reported in a previous clinical trial conducted in Hong Kong. Thus, in this study, a 24-month increment of 1.6 new decayed root surfaces and a standard deviation of 2 was anticipated. In order to show that a 50% difference in mean caries increment between the test and control groups was statistically significant using a 2-sample t-test at a 5% significance level and at an 80% power, a sample size of 100 subjects in each group was required. Allowing for a drop-out rate of 20% over 24 months, a total of 480 subjects (120 in each group each site) were needed.