Plan reference checks and requests.
Not everyone asked will agree to act as a referee, so approaching exactly the number required reliably leaves you short. Dividing the requirement by the agreement rate gives the number to approach, and adding a second response cycle where a shortfall is expected gives a realistic completion time. Building in that buffer up front is what avoids a delayed start date over a missing reference.
Referee outreach with attrition
Referees to approach = ceiling(required / agreement rate); expected agreements = approached x agreement rate; days to complete = response days (+ a second cycle if short) + 2 collation days
Reference practices and what an employer may lawfully disclose vary by jurisdiction. Some employers only confirm dates and job title as a matter of policy.
One or two more than required. Approaching five for three references covers the person who has left the company, the one on leave, and the one who agrees and then does not respond to the employer's request.
Always. An unexpected reference request produces a slow, lukewarm response at best, and a refusal at worst. A short conversation beforehand also lets you tell them which aspects of your work matter for this role.
Almost always a referee who has agreed but not prioritised the request. A polite prompt after four or five working days resolves most of it, which is why one extra response cycle is built into the estimate.