PAPP101 - S06: Mortality statistics and standardisation

Basic measures of mortality (cont.)

The infant mortality rate (IMR)

The infant mortality rate (IMR) is a very common indicator of mortality in a population, and the reduction in this indicator is one of the more important Millennium Development Goals. It is defined as the ratio of the number of deaths of infants (that is, under the age of 1 year), D₀, to the number of live births, B, in the year, multiplied by 1 000:

$$IMR=\frac{d_0}{B}.1000$$

Exercise

Using your knowledge from PAPP101_S03 , is this a true rate?

	a)	Yes
	b)	No
Check your answer
Please select an answer No – it is not a rate, but a risk or a probability because the denominator is those entering exposure (i.e births), not the person-years exposed to risk. In addition, the IMR, as defined, represents an approximation to the risk or probability, as the deaths recorded under the age of 1 in any year will include the deaths of some children born in that year, as well as the deaths of some children born in the previous year, but who were still under the age of 1 in the year under consideration. Thus, there will be a mismatch between the numerator and denominator, which in turn means that the measure is only an approximation to the risk of infant mortality. If the number of births in any year is not changing rapidly, however, this measure will closely approximate a ‘true’ infant mortality rate. Correct – it is not a rate, but a risk or a probability because the denominator is those entering exposure (i.e births), not the person-years exposed to risk. In addition, the IMR, as defined, represents an approximation to the risk or probability, as the deaths recorded under the age of 1 in any year will include the deaths of some children born in that year, as well as the deaths of some children born in the previous year, but who were still under the age of 1 in the year under consideration. Thus, there will be a mismatch between the numerator and denominator, which in turn means that the measure is only an approximation to the risk of infant mortality. If the number of births in any year is not changing rapidly, however, this measure will closely approximate a ‘true’ infant mortality rate.

The timing of infant deaths over the first year of life is highly non-linear. The majority of infant deaths occur in the first few hours, days or weeks of life – especially in developed countries where many causes of infant death are avoidable. In developing countries, significant numbers of children may still die after the first month of life, especially where unsanitary conditions or tropical diseases (e.g. malaria) are common.

To assist policy makers, and to allow for more careful tracking of infant mortality trends, the infant mortality rate is commonly disaggregated into measures of neonatal and post-neonatal infant mortality:

$$IMR=\frac{d_0}{B}.1000=1000.\frac{{}_{1∕12}D_0}{B}+1000.\frac{{}_{11∕12}D_{1∕12}}{B}=$$

= neonatal mortality rate + post-neonatal mortality rate.

where _1⁄12D₀ represents the number of deaths in the first month of life and _11⁄12D _1⁄12 represents the number of deaths between age 1 month and 1 year.

Since the majority of neonatal deaths occur shortly after birth, the neonatal mortality rate (that is the mortality rate in the first month of life) is sometimes further disaggregated into two components, the early neonatal mortality rate (based on deaths in the first week of life) and late neonatal mortality (based on deaths after the first week, but before the end of the first month, of life).

A concept related to early neonatal mortality is that of perinatal mortality, which includes stillbirths along with the deaths in the first week of life in the numerator.