Some mortality rates are not truly rates, even though they may be called a morta- lity "rate." These include rates which use live births as a denominator, such as the UNICEF under-five mortality rate, the infant mortality rate and the maternal mortality rate.
The under-five mortality rate, or U5MR as the term is most commonly used, is the probability of dying before the age of five, usually expressed per 1,000 live births. The U5MR cannot be calculated
directly from data on births and deaths by age in a single year because the deaths, for example, of four-year-olds occur to children born four to five years before the occurrence. U5MR can be cal- culated using life table methods, from birth history data (the recording of the date of each birth and the age of death for those that have died), or using indi- rect methods that are beyond the scope of this manual. This measure of childhood mortality is used most frequently by UNICEF and others who conduct large health assessment surveys.
Comparison of the two measures of under-5 mortality rates
Both U5MR and ASMR-U5 measure the mortality risk for children under 5 years of age, but the two indicators express the risk differently.
• In the case of U5MR, the risk is expressed as the cumulative probability of dying before age 5 years in a hypothetical group of 1,000 births.
• In the case of ASMR-U5, the risk is expressed relative to the mid-interval population as with the crude and other age-specific mortality rates.
Because U5MR expresses risk over 5 years, whereas ASMR-U5 expresses risk per year, U5MR is often almost five times as large as ASMR-U5.
As each measure (U5MR and ASMR-U5) has its uses and its advocates, there is no overwhelming reason to recommend one over the other; they are two different ways of expressing much the same data. Since relief agencies tend to be more familiar with age-specific mortality rates, which are derived in the same way as the CMR (counting deaths during a period of time and dividing by a population denominator), that method will usually be the one used in emergency assessments. Moreover, the usual nutrition and mortality assessment survey done in emergency situations does not gather the data necessary to calculate the U5MR; only the ASMR-U5 can be calculated. On the other hand, U5MR is used by UNICEF and others when measuring child morta- lity in stable populations. It is presented as the measure of child mortality in many summary publications, such as the State of the World's Children. To avoid confusion, any reporting on under-five mortality should specifically note whether it is calculated as an age-specific mortality rate or as the probability of dying by the age of 5 years. For WFP purposes, the ASMR-U5 should be used.
Infant Mortality Rate (IMR)*
The infant mortality rate (IMR) is the num- ber of deaths in children under 1 year of age in a given period divided by the num- ber of live births in the same time period. The infant mortality rate allows assess- ment of the rate of death in the most vul- nerable age group - children less than 1 year of age. It often rises earlier and faster in the face of poor health and nutrition than other mortality rates.
The infant mortality rate also is described as a ratio since many of the children who die and are recorded in the numerator were born before the beginning of the time period, and thus their birth is not recorded in the denominator. If the time period is 1996, for example, then a six-month-old child dying in March 1996 would have been born in 1995; her death would add to the numerator but her birth would not be added to the denominator. Likewise, a child born late in the year would still be at risk of dying under the age of one for much of the following year.
The difference between a ratio such as the infant mortality rate and the usual mortality rate can also be seen another way. The infant mortality rate calculates the chances that a live-born infant will die before his first birthday - it is a cumu- lative incidence. The denominator is the population at the beginning of the time interval of interest. A normal mortality
rate gives the average risk of dying dur- ing the time interval for a person in a population and represents a true rate. It uses as the denominator the average population during the time interval, or mid-interval population.
Maternal Mortality Rate (MMR) **
The maternal mortality rate (MMR) uses as the numerator all deaths of pregnant women or pregnancy-related deaths with- in 42 days of the end of pregnancy. The denominator is live births. Maternal deaths are often a relatively rare cause of death and, as such, the rates should be calculated only for relatively large popula- tions (more than 1,000,000). Random variation in the maternal death rate calcu- lated in small populations with few births may be misinterpreted as significant trends, when they are not actually so. The maternal mortality rate is critical to determine the need for antenatal and obstetric services. Although the actual proportion of all deaths in a population resulting from pregnancy-related causes is often small, the effects of a woman of childbearing age dying are often much greater for her family and the society than the deaths of others in the population. Therefore, in a certain country, if there were 34,459 births in 2004 and 78 mater- nal deaths, the maternal mortality rate would be 226.4 deaths per 100,000 live births.
DEFINING AND MEASURING MORTALITY CHAPTER
2
*
CROSS-SECTIONAL SURVEYS FOR