Statistical analysis
We calculated sample-size by two independent
proportions.17, 18 Using estimated 6-month improvement
rate of 83% with LNG-IUD insertion and 71% with hysteroscopic niche
resection based on our pilot study, and assuming an alpha level of 0.025
for a one-sided test, we calculated that a sample of 68 cases who
underwent therapy would result in 80% power (1:1 ratio). The sample
size was increased to 76 to allow for a 10% rate of loss to follow-up
over the 12-month period.
All statistical tests were conducted using IBM SPSS 23(SPSS Inc.,
Chicago, IL, USA). We used both ITT and per protocol analyses. For the
primary outcome (effective rate)was based on the ITT analyses. The mean
± standard deviation was used to expressed the continuous data that were
normally distributed, the differences between groups were tested using
the student t test. Otherwise median and interquartile range (IQR) were
used and tested with Mann-Whitney U
test.
Categorical variables were compared by the χ 2 test;
if more than 25% of cells were found to have an expected value of
between 1 and 5, Continuity Correction Chi-Square test was used instead;
if more than 25% of cells were found to have an expected value of less
than 1, Fisher’s exact test was used. The mix linear model was used to
compare the effective rate of spotting symptoms repeatedly measured
within 1 year after surgery between two groups and the spotting days
between each follow-up time points and baseline.
Linear-by-linear
association test was used to calculated the P for trend to expressed the
efficacy change over time. Logistic regression analysis was used to
analyze the effect of the TRM and also of age on spotting and total
bleeding days at each follow-up point. The Statistical significance was
determined by a p value of <0.05.