Probably the best thing you can do is start collecting data around the quality of the hires, and who interviewed them/hired them/etc. You should see that some p

Probably the best thing you can do is start collecting data around the quality of the hires, and who interviewed them/hired them/etc. You should see that some people are better at hiring than others (yes, this is a skill). Start by collecting the data you already such as annual reviews and fired employees. You should start doing is having every hiring manager bucket rank their hires after 3 months, 6 months, 1 year. Is their recent hire a star, above average, average, below average or poor? Would you hire this person again? Keep in mind, that this will also reflect how well you onboard engineers. If someone has a really good onboarding process then that may transform the average engineer to an above average engineer. Likewise a poor onboarding experience can make an above-average engineer into a poor engineer.