data describing an individual’s socioeconomic circumstances have not been, or cannot be, collected directly (Danesh et al. 1999). The clusters were ranked on the basis of the con- tinuous UADI scores and categorized into quintiles divided at the 20th, 40th, 60th, and 80th percentiles, such that class I was assigned to the 20% least disadvantaged urban areas and class V the 20% most dis- advantaged urban areas. The ranks indicate how a neighborhood compares with all the other neighborhoods and are easily interpre- table. Normalized sample weights provided in the DHS data were used for this analy- sis, employing Stata version 10 (StataCorp, College Station, TX, USA) to adjust for non- response and enable extrapolation of findings to the general population. Individual-level explanatory factors. Potential confounders were grouped into child- and mother-level demographic and socioeconomic characteristics and included a) sex of the child, categorized as male and female; b) birth order and interval between births, created by merging “birth order” and “preceding birth interval,” classified as first births, birth order 2–4 with short birth interval (< 24 months), birth order 2–4 with medium birth interval (24–47 months), birth order 2–4 with long birth interval (≥ 48 months), birth order ≥ 5 with short birth interval (< 24 months), birth order ≥ 5 with medium birth interval (24–47 months), and birth order ≥ 5 with long birth interval (48 months); c) moth- er’s age, grouped as 15–18, 19–23, 24–28, 29–33, and ≥ 34 years of age; d) marital status, categorized as single, married, and divorced; e) mother’s education, categorized as no edu- cation, primary, and secondary or higher edu- cation; f) mother’s occupation, grouped as professional/technical/managerial, clerical/ sales/services/skilled manual occupations, and not working; and g) wealth index, catego- rized into quintiles as poorest, poorer, middle, richer, and richest. Statistical analyses. Trend in urban under-5 mortality rates between 1986 and 2003. The probability of child deaths among children < 5 years of age was directly estimated from the 1990, 1999, and 2003 Nigeria DHS birth history data. The urban population pat- tern for 1983–2003 was derived from the United Nations Department of Economics and Social Affairs (2004). Multilevel logistic regression modeling. The data were analyzed using MLwiN ver- sion 2.10 (Rashbash et al. 2008). We fitted a multilevel model with binomial, penalized quasi-likelihood procedures and second- order linearization (Goldstein 2003). We used a three-level multilevel logistic regres- sion analysis with 2,118 children (level 1), nested within 1,350 mothers (level 2), who were in turn nested within 165 communities Urban area disadvantage and under-5 mortality (level 3). Four sequential models were fitted to a) examine the effect of no predictor vari- ables in the fixed part, but only the intercepts in the random part so as to present a baseline for comparing the magnitude of contextual variations in under-5 mortality risks in sub- sequent models (model 0); b) examine the association between under-5 mortality and urban area disadvantage (model 1); c) adjust for child-level characteristics (model 2); and d) simultaneously adjust for urban area dis- advantage and both child- and mother-level characteristics (model 3). The measures of association (fixed effects) for each of these models were expressed as odds ratios (ORs) and their 95% confidence intervals (CIs). Measures of variation (random effects) were expressed as variance partition coefficient (VPC) and percentage change in variance (PCV). VPC expresses the propor- tion of the individual differences in the risk of under-5 deaths (i.e., individual variance) that is at the community level. A VPC different from zero is indicative of significant differences in under-5 mortality risks between mothers and communities. We estimated the PCV to evalu- ate how much of the variance in the first model is attributable to differences in individual char- acteristics (Merlo et al. 2007). The significance of the random variation at each level was tested with the Wald test, and p-values were based on a chi-squared distribution. The deviance infor- mation criterion (DIC) was used as a mea- sure of how well the different models fitted the data. Lower values indicate a good model fit relative to the number of parameters in the model (Spiegelhalter et al. 2002). Results The urban under-5 mortality rate in Nigeria declined from 74 per 1,000 in the period 1979–1983 to 52 per 1,000 in 1984–1988. It then increased to 142 per 1,000 in the period 1999–2003. Urban population in Nigeria showed a steady increase from about 27,000 Urban population per 1,000 U5MR per 1,000 160 140 ;00 ,000120 ,it/10on tlti/y1a100 lau ro 80 p m po veﬁ 60 an r- rbU nude 40 20 0 in 1986 to about 61,000 in 2003 (urban pop- ulation here refers to the de facto population living in areas classified as urban according to the criteria used by each area or country) (Figure 1). Table 1 presents the distribution of the independent variables by UADI. Children in the most disadvantaged UADI quintile (class V) most frequently were male, were of high birth order and medium birth inter- val (order ≥ 5 and 24–47 months), and had mothers who were younger (24–28 years of age), married, and uneducated, worked as clerical/sales/services/ skilled manual employ- ees, and were in the poorest household wealth quintile. On the other hand, a higher propor- tion of children in the least disadvantaged UADI quintile (class I) were male, were of low birth order and medium birth interval (order 2–4 and 24–47 months), and had mothers who were older (≥ 34 years), mar- ried, and educated at the secondary or higher level, worked as clerical/sales/services/skilled manual employees, and were in the richest household wealth quintile. Figure 2 illustrates the association between under-5 mortality and UADI and shows that under-5 mortality varied according to urban area disadvantage, with moderate increases in under-5 mortality risk associated with increas- ing urban area disadvantage. This means that the risks of dying were higher for children of mothers residing in increasingly disadvan- taged urban areas. Table 2 presents the results of the multi- level analysis for the association between urban area disadvantage and under-5 mortal- ity. Model 0 gives an indication of the amount of spatial clustering of under-5 mortality and indicates that the community-level variance is significant (τ = 0.273, p = 0.014), whereas the mother-level variance remains nonsig- nificant, suggesting some clustering of moth- ers of children with similar risks of under-5 deaths within disadvantaged communities. 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 Year Figure 1. Trends in urban under-5 mortality rates (U5MR) and urban population in Nigeria, 1986–2003. Urban under-5 mortality rates (per 1,000) were directly estimated from the 1990, 1999, and 2003 Nigeria DHS birth history data. Urban population data were from United Nations Department of Economics and Social Affairs (2004). Environmental Health Perspectives • volume 118 | number 6 | June 2010 879