I was just interviewing a large number of people as part of an acquisition. After the interviews were done, the interviewers all went out to dinner and discussed the day's events. While at dinner, the topic of employee retention came up. It was during this discussion that I stated my "Law of Employee Retention." The law is simple and is stated in terms of "half-life", which means the time in which 50% of the employees will leave.
An engineer with a spouse whose family is located "far away" has a retention half-life of 5 years.
"Far away" means that the spouse's family is far enough away that plans need to be made to visit them.
I have over 30 years of experience with this rule. I started gathering my data when I was a recruiter with HP. It even worked for me and my wife. I lasted five years with HP before my wife wanted to get closer to her family.