Alissa at All Metrics, All The Time says:
"wouldn’t it be more effective to see why the people who came on board did so? If you’re measuring a positive metric and getting a negative result, while not necessarily a good sign, it will give you a better view of what is going on in your hiring process and what you can do to improve. "
She and Anuradha have been having a fascinating conversation regarding HR Metrics.
Personally, I think discovering the "why" behind any action leads to true learning. However, looking at "retention" metrics might have more positive results and make you look at much more options than if you look at "attrition" metrics.
For example, some HR managers I know conduct informal "stay interviews" to get at reasons why high performers stay, and discovering that data leads to much better processes for retention of people who are more valuable to the organization.
It's the same philosophy that fuels Appreciative Inquiry as a OD tool- focusing on what works and strengthening that, rather than diagnosing what is "wrong".