Quality of hire
Quality of hire attempts to measure how good your hires turn out to be, typically combining performance ratings, ramp time to productivity and retention beyond the first year. It is the most important recruiting metric and the hardest to measure credibly.
Most implementations rely on a manager rating at 6 or 12 months, which is subjective and often more a measure of the manager-report relationship than of the hire.
A more tractable approach is to track a small number of harder signals — first-year attrition, time to full productivity, promotion within two years — and accept that no single number captures it.