Attrition (or turnover) is the share of your team that departs in a given window, usually reported annually. In support it runs high, and it is expensive twice over: the direct cost of hiring and the hidden cost of the months a replacement spends becoming competent.
The formula is departures divided by average headcount over the period.
What it hides: a single rate blurs regretted vs. non-regretted exits and hides where the bleeding is — much of support attrition happens in the first year, so a stable-looking annual figure can mask a broken onboarding. Read it with tenure and first-90-day survival, not on its own.