 Solution

The groups were not formed by randomization. Their background health, access to care, clinic, country and reasons for selecting a delivery method can predict both the method and the outcome, creating confounding and selection bias. Moreover, the label describing the reference delivery may be determined after labour: patients whose attempted nonoperative delivery develops a complication can move into an operative category, leaving a selected uncomplicated reference group. Comparing intended strategies would avoid treating this post-baseline classification as a baseline assignment.

A useful comparison would therefore align eligibility and follow-up time, compare intended delivery strategies among patients genuinely eligible for both, and account for sufficient pretreatment predictors. Logistic regression cannot create comparable patients where the positivity assumption fails or remove unmeasured confounding.