Let's do a thought exercise. Say you have a population. This population suffers a shock. Roughly 10% of the population flees after the shock. Now think about this. Would that 10% that fled be healthier or less healthy than the 90% that stayed behind?
Which would have a better chance of being able to flee/relocate the wealthier healthier population or the poorer sicker population?
Now if all other things are equal what happens to the death rate of a population that has more of a concentration of sicker less wealthy people? Even if you factor in for population change?
You now have a different population make-up after the shock than you had before and thus the numbers on that population would be different. Trying to compare the numbers after the shock to the numbers before the shock is a waste of time because your inputs are now different.
You are no longer comparing apples to apples but apples to grapes.
