 10. Sample size estimation and statistical analysis: It is projected that the standard deviation of morning blood pressure in the enrolled hypertensive patients is 12 mm Hg, and amlodipine, compared with nifedipine GITS, reduces morning blood pressure at 4 months of follow-up by 4 mm Hg. If α0.05 and power90, the study will require a sample size of 190 hypertensive patients per group to detect the projected difference in the morning blood pressure 4 am-8 am. After accounting for 5 of drop out rate and 20 patients on add-on therapy, the sample size for each group is 250. Thus the number of required subject for the whole trial is 500. 