The study was conducted in urban and rural communities
in Kashgar, the westernmost city of China, whose
population is 92% ethnic Uygur. The drinking water
fluoride level in this region is 0.3 ppm. The minimum
sample size (n =267) was calculated before study initiation,
based on an ECC prevalence of 60% (estimated
from the ECC prevalence in the Urumqi population),
with a margin of error of 5%, a 95% confidence level,
and allowances for a 15% non-response rate, resulting in
a required sample size of 313. Initially 730 participants
were enrolled in the study. Twenty-nine parents refused
to participate in the study because of lack of time. We
obtained 697 valid questionnaires; to avoid absence from
kindergarten, 27 of these children did not undergo oral
examination and were thus excluded. The final analysis
included 670 participants, yielding a 92% response rate.