I agree with Menaka that exit interviews after resigning can be very valuable. The person will likely be in new employment so not as concerned about the information they provide. They will also have a more balanced view of their past employer having stepped away. However, post employment exit interviews are more difficult to obtain as contact details may have changed and people may be less willing to give their time. We have found that conducting the interview between 1 and 3 months after employment ends works well as the information provided is still current. Employee agreement to conduct a post-employment interview will also help to secure the interview.