Editor—Kotaska outlines the importance of care providers' skill and the experience of a unit when assessing non-pharmacological treatments such as vaginal breech delivery in randomised trials.¹ We agree that evaluation of non-pharmacological treatment raises specific methodological issues, including the skill of care providers.²

Care providers are part of the intervention to be tested, and having highly skilled or experienced care providers in one arm and low skilled or less experienced care providers in the other could lead to bias. Equally, bias can occur when care providers have more experience in performing one of the interventions tested than the other. However, appropriate methodological planning of randomised trials could circumvent this bias. To allow the surgical procedure to be assessed in the context of the skills required to achieve it, care providers participating in a surgery trial could be trained and selected only if they achieve set standards,³ selected according to their experience of the procedure,⁴ or patients could be randomised not to operations but to care providers, who would deliver their treatment of preference.⁵

Kotaska's article was an interesting example of potential bias linked to care providers' experience; the author generalises when concluding that complex procedures are poorly amenable to the methods of large multicentre randomised trials. Condemning all multicentre randomised trials that assess complex interventions when considering one imperfect randomised controlled trial seems as inappropriate as defining a new standard of care for vaginal breech delivery based on a single trial that is potentially biased.