HR people in their desire to know why people are disengaged enough to quit organizations often conduct exit interviews - to talk to employees who have resigned - and trying to understand the reasons why they quit. (Some organizations also conduct what they call 'stay interviews' trying to understand why people are staying!)
However every HR professional who has conducted such an interview would know that more of then not, these interviews would hardly unearth anything really useful.
When an employee is spending his/her last few days in the organization, to expect him/her to be absolutely candid and frank is unrealistic. People want to get their dues settled and don't want to burn too many bridges. In many countries the next job might be dependent on the background verification that the existing organization gives.
However, what HR people can do is to call when the employees have joined the next organization. Say, after a month or so. That is the time when an ex-employee can be candid and has no real reason to pull any punches.
Or better still, get a third party agency to make that call.
