= 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>.
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