Sample size
As considered the primary outcome, sample size calculation was conducted to compare the IUD displacement rate between IUD insertion using the conventional technique and IUD insertion using the new technique. The calculation was performed based on comparing two proportions from independent samples using the Fisher Exact test, the α-error level was fixed at 0.05, and the power was set at 95%. Accordingly, the optimum minimal sample size should be 420 participants in each group.