Sample size estimation was based on figures derived from a
well-designed randomised controlled trial (Ford et al. 2004,
p. 42). In this trial, serum phosphorus level was
166 043 in the experimental group and 214 054 in
the control group after the intervention. Using power of
90% and two-tailed significant level of 5% led to 19 participants
in each group. The formula used was as follows