Learning curves. (a) Complication hazard for trainee and senior according to the rank (number of THA) and the time of follow-up (in days). The trainee surgeons had a spike of complications for their 10 first patients at 500 days post-operatively. (b) Comparison of cumulative probability of post-operative complication at 800 days between senior and trainees. The learning curve of the trainee surgeons is faster that of the senior. The trainee surgeons joint the learning curve of the senior after 10 THA. (c) Learning curve of operating time in senior group, with a steady state around 70th case for the senior surgeon. Rank = number of patients operated (experience).