Statistical analysis
Sample size calculation was performed by using data from van
Bavel’s study, which evaluates the risk of OASI: the true risk of OASI
in primiparous women with mediolateral episiotomy is 3% and 26 % in
instrumental delivery.4 A sample size of 33 for each
group was calculated based on these data and a one-sided t -test
(p < 0.05) was estimated to have 80% power to reject
the null hypothesis that uterine fundal pressure does not increase the
risk of OASI as much as forceps delivery. Statistical analysis was
performed after normality testing (histogram analysis and/or
Kolmogorov-Smirnov test) using IBM SPSS, version 21 (IBM Inc., Armonk,
NY). Student’s t-test was used for the comparison of normally
distributed variables, while the Mann-Whitney U-test was used for
non-parametric variables, and Fisher’s exact tests were used to compare
categorical variables. Variables were then considered jointly in a
multivariable stepwise forward multivariable logistic regression model
to predict OASI.