Statistical analysis
In absence of previous pilot study to calculate the sample size for the
prevalence of preeclampsia in women exposed to H2S
pollution, we have considered that incidence of preeclampsia of women in
non-exposed areas was 6%. Assuming alpha=0.05 and power of 85% with a
non-exposed/exposed ratio of 4, a sample of 3020 women was planned to be
able demonstrate a 3-point difference between non-exposed/exposed women.
For all descriptive analyzes, assumption of normal distribution of the
data was analyzed. Mean and standard deviation, or median and
interquartile range were reported when appropriate. Categorical
variables were presented as absolute values and percentages. The
following tests were used for group comparisons depending of the
conditions of application: Student t-test, Mann-Whitney test, Chi-square
test or Fisher’s exact test. Kaplan-Meier analysis was conducted to
describe preeclampsia free survival time and compared using the log-rank
test. The level of statistical significance was set at p<0.05.
All statistical analyses were conducted using SAS software 9.4 for
Windows (SAS Institute, Cary North Carolina, USA).