Who should assess competence — the line supervisor or an independent assessor?

The line supervisor knows the work and sees the person daily. They also own the schedule, and signing someone off as not yet competent creates a resourcing problem they then have to solve. That is a real conflict of interest, not a theoretical one. An independent assessor removes the conflict and adds cost, delay, and someone who may not know the site. Splitting the difference — supervisor assesses, independent assessor verifies a sample — is what most standards expect. How big is your sample, and has verification ever actually overturned a sign-off?