Mortality rates for children
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An important caveat:
There are actually two very different mortality rates, both of which are called "under-5 mortality rate." They are both measures the risk of death in children less than 5 years of age, but they are calculated differently and result in very different numbers.
The first is an age-specific mortality rate for children less than 5 years of age. It is calculated:
or number of deaths per 1000 per year or number of deaths per 1000 per month.
Note that this is a true rate, with the numerator and denominator both being children less than 5 years of age.
The second "under-5 mortality rate" is defined as the probability of dying between birth and the fifth birthday expressed per 1,000 live births. It is not a true rate, but rather a cumulative incidence of mortality during the first 5 years of life. It is usually calculated using life table techniques from survey questions asking mothers about children born within 5 years before the survey.
In non-emergency situations, this cumulative incidence of mortality is the more measure; it is the measure of child mortality in major publications such as UNICEF's State of the World's Children. Usually, public health workers in stable populations will be most familiar with this indicator, and will be surprised to see epidemiologists expressing child mortality as an age-specific mortality rate.
On the other hand, humanitarian relief workers may be less familiar with this cumulative incidence of mortality because measuring it in surveys is quite complex and requires expertise usually not easily available in emergencies. Therefore, below is a bit more explanation:
In this diagram, we imagine a group of 1000 babies born exactly 5 years ago. These babies are followed over time. Some will die before reaching their fifth birthday. The cumulative incidence of mortality is the number of these 1000 babies who will die before their fifth birthday divided by the 1000 babies who were in the cohort in the beginning.
As you can see, the second under-5 mortality rate counts deaths for 5 years' time and is, therefore, about five times higher than an annual age-specific mortality rate for children less than 5 years of age. However, because the risk of death is not the same throughout the first 5 years of life (it is much higher in the first year of life, then steadily drops until the fifth birthday), the cumulative risk cannot be divided by 5 to obtain the age-specific annual mortality rate.
When someone quotes an "under-5 mortality rate," you must always be clear which rate they are quoting. UNICEF Multiple Indicator Cluster Surveys (MICS) and Demographic and Health Surveys (DHS) can provide excellent baseline data against which to compare mortality rates derived from an emergency assessment survey; however, they measure the cumulative risk of death in the first 5 years of life and cannot therefore provide a direct comparison to an age-specific mortality rate for children less than 5 years of age measured in the assessment survey.