Measuring whether onboarding programmes really work

We redesigned onboarding for new engineers at our company, adding mentoring and a structured first month. Feedback forms are positive, but leadership wants evidence that people become productive faster and stay longer.

We now track time to first independent task, mentor feedback at thirty and ninety days, and retention after one year. The numbers are still small, so it is hard to draw conclusions. What indicators have you found meaningful when evaluating onboarding, and how long did it take before trends became clear?