The purpose of an exit interview is to help you understand why your employee has decided to leave your company. The picture you build up from successive exit interviews will identify persistent problems (with work, management or pay) so you can consider taking remedial action to prevent future staff losses.
Most leavers stress positive reasons for their decision: a new challenge, an opportunity too good to miss, a bigger salary. They are unlikely to criticise the company or their manager when they need a good reference for their new employer. Managers, reluctant to hear criticism, readily accept the superficial explanation and an opportunity to learn is missed. To overcome this, try using someone other than the leaver’s immediate manager to conduct the interview. Ask probing questions such as:
Alternatively, use a questionnaire posted to the leaver after the termination date.