Background Under-5 mortality rate (U5MR) is the probability of a child born in a specific year or period dying before reaching the age of 5. Nigeria has a high rate of U5MR and accounts for a significant proportion of the high U5MR in sub-Sahara Africa. There are differences in health and social practice in the different states in Nigeria, coupled with the differences in developmental priorities of each State government. There is therefore the need to identify the states with high and low U5MR, to further explore the risk factors and make recommendations for planning. This study investigates variation in U5MR in Nigeria using Shewhart's theory of variation and control charts.

Methods We used data from the birth histories included in the 2008 Nigerian Demographic and Health Survey to estimate U5MR using a synthetic cohort life table. We plotted control charts of the proportion of under-5 mortality for the 37 states (included federal capital Abuja) in Nigeria. The Local Indicators of Spatial Association was used as a measure of the overall clustering and is assessed by a test of a null hypothesis.

Results On average, more than 1 in every 10 children born in Nigeria (159 per 1000 live births) does not survive to their fifth birthday. Kwara and Osun states had the lowest U5MR (less than 60 per 1000 live births), while Jigawa, Kano, Sokoto, Niger and Adamawa states had the highest U5MR (more than 200 per 1000 live births). There is a wide variation in the U5MR between the 37 states. The funnel plot identifies 27 (73%) states within the 99% control limits indicating common-cause variation. Four states were above the upper control limit (higher than the average) and six states were below the lower control limit (lower than the average), indicating special-cause variation.

Conclusions U5MR is high in Nigeria; the rates are in three digits. There is a wide variation in the U5MR in Nigeria, with 27% showing evidence of special-cause variation which merits further investigation to identify possible causes. However, the vast majority of states (73%) are consistent with common-cause variation