Methodology

Definition of indicators

Under-five mortality rate: Probability of dying between birth and exactly five years of age, expressed per 1,000 live births.

Infant mortality rate: Probability of dying between birth and exactly one year of age, expressed per 1,000 live births.

Neonatal mortality rate: Probability of dying during the first month of life, expressed per 1,000 live births.

Data sources and methodology

Generating accurate estimates of child mortality poses a considerable challenge because of the limited availability of high-quality data for many developing countries. Complete vital registration systems are the preferred source of data on child mortality because they collect information as events occur and they cover the entire population. However, many developing countries lack fully functioning vital registration systems that accurately record all births and deaths. Therefore, household surveys, such as the UNICEF-supported Multiple Indicator Cluster Surveys and the US Agency for International Development–supported Demographic and Health Surveys, are the primary sources of data on child mortality in developing countries.

The IGME updates its child mortality estimates annually after reviewing newly available data and assessing data quality. The IGME seeks to compile all available national-level data on child mortality, including data from vital registration systems, population censuses, household surveys and sample registration systems.

Data from different sources require different calculation methods and may suffer from different errors, for example random errors in sample surveys or systematic errors due to misreporting. As a result, different surveys often yield widely different estimates of under-five mortality rate for a given time period. In order to reconcile these differences, the Technical Advisory Group of the IGME has developed methods to fit a smoothed trend curve to a set of observations and to extrapolate that trend to a defined time point, in this case 2011. To estimate the under-five mortality trend series for each country, a statistical model is fitted to data points that meet quality standards established by the IGME and then used to predict a trend line that is extrapolated to a common reference year, set at 2011 for the estimates in this round. Overall, infant mortality rates are generated by transforming under-five mortality rates based on model life tables. Neonatal mortality rates are produced using a statistical model that uses under-five mortality rates as an input. These methods provide a transparent and objective way of fitting a smoothed trend to a set of observations and of extrapolating the trend over 1960 to the present. Details on these methods can be found in the PLOS Medicine Collection on Child Mortality Estimation methods (www.ploscollections.org/childmortalityestimation).

Changes in the estimation process

The IGME continually seeks to improve its methods and may introduce changes from one year to the next. This year, the IGME produced sex-specific estimates, added uncertainty intervals for the estimates and implemented other changes. Details can be found in the Annex – Technical Notes.

In addition, a substantial amount of newly available data has been incorporated: data from more than 20 of the most recent surveys and censuses conducted since 2009 for more than 20 countries, new data from vital registration systems for about 70 countries and data from more than 50 surveys and censuses conducted before 2009 for more than 20 countries.

The increased data availability has substantially changed the estimates for some countries from previous editions partly because the fitted under-five mortality rate trend line is based on the entire time series of data available for each country. Furthermore, model life tables and a statistical model are used to derive estimates of infant and neonatal mortality rates based on under-five mortality rates. Therefore, the estimates presented in this round of estimation may differ from and are not necessarily comparable with previous sets of IGME estimates or the most recent underlying country data.

More details on the data used in deriving estimates are available in CME Info (www.childmortality.org).

Key references