Number of excess deaths
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Many surveys which measure mortality in emergency-affected populations present the number of excess deaths which have occurred since the emergency began. This is hypothetically the number of deaths caused by the emergency itself, or rather the number of deaths that would not have occurred had the emergency not happened.
Calculation of excess deaths requires a baseline, pre-emergency mortality rate, which is often difficult to obtain. The difference between this baseline and the mortality rate found in the survey is then multiplied by the population size to obtain the number of excess deaths. The number of excess deaths is heavily dependent on what pre-emergency mortality is assumed and on the estimated population size.
In a population of 10 million people, a survey was done to measure mortality since an emergency began one year ago. The pre-emergency baseline crude mortality rate was 6 deaths per 1000 per year. The crude mortality rate estimated from the survey for the past year was 8 deaths per 1000 per year. How many excess deaths have occurred since the emergency began one year ago?
For a much more complete discussion of this and other issues in measuring mortality rates, see Checchi F and Roberts L. Interpreting and using mortality data in humanitarian emergencies: a primer for non-epidemiologists. Humanitarian Practice Network No. 25. Overseas Development Institute, 2005. (Click here to open this docuent.)