There are many difficulties in measuring mortality with respect to those dimensions. A test memo says the follownig:
1. It is difficult to record everyone's exact social, economic and cultural factors
2. If a death occurs, it is often difficult to ascribe the death to some specific factor because often it is unknown or there was a chain of events (in which many of the above factors played a role).
I fully understand what is meant by the above points. However, the memo mentions a 3rd point (which I require some clarity on):
3. (The main difficulties in measurements is) Relating within each axis of classification, deaths to the specific population at risk