Retention is keeping the people a business has already hired and trained.
What replacing someone costs
Recruiting time, the vacancy itself, and the period a new person is paid full wages while producing less than full output. In skilled trades that ramp is months, not weeks.
Against a competitor's offer of a few per cent more, the arithmetic usually favours matching it. The replacement cost is real and is rarely calculated, while the pay rise is visible immediately, which is why the decision is routinely made the wrong way round.
What actually drives leaving
Pay is the stated reason more often than it is the real one. The recurring causes in service businesses:
The immediate supervisor. More people leave a manager than leave a company.
Scheduling. Unpredictable hours, being called out repeatedly, and last-minute changes wear people down faster than the pay compensates for.
Equipment and vehicles. Being sent out under-equipped is read as an assessment of the person's worth.
No route forward. Skilled people leave when the next step does not exist, particularly where the only progression is to a role the business does not have.
Finding out before they go
The information exists in the business and is rarely asked for. An exit conversation is too late to keep that person, though it is worth having; asking current staff is what changes anything.
Compensation structure
Predictable earnings matter more than headline rate to most people in the trades. A structure where earnings swing with factors outside the person's control is a retention risk regardless of the average — see commission and bonus plans.
Measuring it
Turnover as a share of headcount, read over years rather than months. In a small business one departure distorts a single year, so the trend is the only reliable reading.