Measuring Within and Between Group Inequality in Early-Life Mortality Over Time: A Bayesian Approach with Application to India

04/23/2018
by   Antonio P. Ramos, et al.
0

Most studies on inequality in early-life mortality (ELM) compare average mortal- ity rates between large groups of births, for example, between births from different countries, income groups, ethnicities, or different times. These studies do not mea- sure within-group disparities. The few studies that have measured within-group variability in ELM have used tools from the income inequality literature. We show that measures from the income inequality literature, such as Gini indices, are inappropriate for ELM. Instead we develop novel tools that are appropriate for analyzing ELM inequality. We illustrate our methodology using a large data set from India, where we estimate ELM risk for over 400,000 births using a Bayesian hierarchical model. We show that most of the variance in mortality risk exists within groups of births, not between them, and thus that within-group mortality needs to be taken into account when assessing inequality in ELM. Our approach has broad applicability to many health indicator

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1 Introduction

Inequality in early-life mortality (ELM) is a fundamental dimension of social inequality (Moser et al. 2005; Stuckler et al. 2010). It is usually defined in by differences in average mortality rates between groups of births characterized by a single demographic, such as race, gender, income or country. In high income countries, national averages of child mortality are less than 10 deaths per thousand births, while these rates can be over 200 deaths per thousand births in low income countries. Within-country disparities in early-life mortality can be just as large, particularly across groups defined by wealth and income (Victora et al. 2003; Sastry 2004; Wagstaff 2000) and race and ethnicity (Brockerhoff and Hewett 2000; Antai 2011; Jankowska and Weeks 2013).

If births within a group have similar mortality risk, quantification of between-group disparities based on group averages is sufficient to characterize disparities in ELM in human populations. If births within a single group have widely varying underlying mortality risk, then within-group variation in mortality risk can be higher than between-group variation. Demographic and epidemiological research suggests that births within a group can have very different mortality risks. Thus we should expect a group of births defined by only one or a few demographic characteristics to have a variety of mortality risks. The few studies that have measured inequality within groups of births find that the variation in mortality risk among individuals from the same group (e.g. births from the same country) is often larger than the difference in group averages (Gakidou and King 2002, 2003). If that is the case, it is important to quantify within-group inequality in mortality in a manner similar to the quantification of income inequality.

Previous work on the measurement of inequality in ELM has borrowed inequality measures from the income inequality literature (Gakidou and King 2002, 2003; Murray 2001) and these measures have been implemented by international organizations (World Health Organization 2000; NIMS et al. 2012)

. However, it is not clear that these measures are appropriate for studying mortality risk. Mortality is measured on the probability scale and thus is bounded inside the

interval. Income, however, is usually defined on the positive real line or, sometimes, on the whole real line. Properties that are appropriate for measures defined on the real line, such as scale invariance, are not appropriate for probabilities. Thus while it is clear that within-group variability in ELM should be quantified, it is unclear how to do so.

Standard demographic decomposition techniques, such as Kitagawa (1955) or Oaxaca (1973), have only been applied to explain average

differences in ELM and other health indicators between populations. They do not quantify other aspects of distributional changes such as variances or quantiles, which are essential to the study of inequality. In fact, two populations with the same average rates of ELM can have very different distributions of mortality risk

(Gakidou and King 2002). Thus, it is important to develop a framework that quantifies and explains differences in distributions of mortality risk beyond mean changes. Currently no methodology measures and explains differences in the distribution of mortality risk across populations in a manner similar to decompositional methods in labor economics (Fortin et al. 2011) or relative distribution methods (Handcock and Morris 1999).

Mortality risk is not an observable quantity, however. We only observe that births are still alive or not at a certain age. To measure and explain inequalities in ELM across births we propose to estimate birth risk using a statistical model. Only then can we proceed to the inequality analysis. To make proper inferences we will need to propagate uncertainty from the estimation stage to the analysis of inequality stage.

In this paper we develop new methods to analyze the distribution of mortality risk with the objective of measuring and explaining health inequality within and across populations. We demonstrate our methodology using two waves of the Demographic and Health Surveys (DHS) with information on more than 400,000 births from India. We estimate the underlying infant mortality risk with associated uncertainty and complex posterior covariance structure using Bayesian hierarchical logistic regression fit via Markov chain Monte Carlo (MCMC). Bayesian estimation and MCMC methods can handle complex models and facilitate the propagation of model and estimation uncertainty into subsequent analysis of inequality. Hierarchical models are a natural choice in our context because cases in our data set are nested in larger sampling units: births are nested in mothers, mothers are nested within sampling clusters, which are nested in districts and districts in states. Our inference target is the empirical distribution of mortality risk both marginally and as a function of covariates.

We make a number of contributions with broader applications to the existing literature inequality in ELM. First, we show that the usual practice of measuring inequality in ELM between groups of births using measures from the income inequality is not appropriate. Thus we suggest that to investigate ELM one needs measures that are specifically developed for mortality.

Second, proper assessment of inequality requires the consideration of within-group variability. We introduce methods that are appropriate for quantifying inequality in ELM within and between groups of births. We introduce summary measures that quantify the overall difference between two distributions. We develop adjustments that parsimoniously summarize differences between distributions of mortality risk. We extend covariate adjustment methods from the demographic literature by expanding the types of comparisons. We propose comparisons between distributions, while most research in demography and public health only compares means or other summary measures. Our methods allow us to separate the impacts of changes in population composition from changes in the covariate-outcome relationship for binary outcomes. This makes it possible to answer questions such as how the distribution of mortality risk in 1995 would have looked if there had been no changes in the distribution of maternal age since 1975. We also develop ANOVA methods applied after Bayesian model fitting to quantify within-and-between group variance in mortality risk. Gakidou and King (2002)

have suggested that most of the variance in mortality risk occurs within groups of births, not between them. We develop methodology to formally demonstrate the truth of this hypothesis. Our methodology directly allows for complex inference targets while allowing for measurement of uncertainty. Inequality measures and ANOVA are examples of new complex inferential targets that benefit from Bayesian inference.

Third, our analysis uncovers several new patterns of inequality in ELM in India. Inequality in ELM in India has been widely studied (Bhattacharya and Chikwama 2012; Singh et al. 2011; De and Dhar 2013; Nidhi et al. 2013; Kumar and Sing 2014; NIMS et al. 2012). The level of aggregation varies in these studies (states, districts, etc.), but great inequality has been documented. However, inequality in ELM at the individual birth level has not been investigated previously. We show that looking at average mortality rates among socioeconomic groups provides an incomplete picture of ELM inequality in India. For example, demographic groups defined by caste, maternal age at the birth of the child and religion explain less than 4% of all variance in ELM risk. Groups defined by quintiles of wealth explain between 7% and 10% of all variance. District has the highest explanatory power but still explains less than 20% of all variance in ELM risk. These patterns are consistent over time and suggest that previous analyses have largely overlooked variability in ELM risk within groups of births. We show that differences over time in the distribution of ELM risk can be summarized by a simple multiplicative shift. And, finally, we show that changes over time in covariate distributions only account for a small fraction of the changes over time in ELM risk.

The paper is organized as follows. In section 2 we show that income inequality measures are not appropriate for studying ELM. Section 3 suggests summary measures and adjustments that are appropriate for assessing inequalities in ELM. Section 6 introduces covariate adjustment methods. Section 7 discusses some of the implications of our findings. Details of the data and the complete model we use for estimating infant mortality risk are presented in the appendix.

2 Problems with applying measures from the income inequality literature to mortality data

Most measures of income inequality have a common form (Firebaugh 2002). For a population of births with mortality risk , income inequality measures take the form

Inequality (1)

where is the average risk in the population. Define which is the ratio of the birth mortality risk to the population average risk; inequality measures (1) are functions of . Three popular measures of inequality are the squared coefficient of variance, Theil indexes, and variance of the logs

(2)
(3)
(4)

and the closely related Gini index

Gini (5)

There are at least three problems in applying (1) or (5) to mortality data. First, in (1) or (5) the average level of income of the population under study is in the denominator. This is not a problem for income, as average income is usually far from zero and income inequality measures are designed to be the same whatever currency or units income is measured in or if income is doubled or halved across the board. Applied to mortality data, these indices divide the expected mortality risk for each birth, , by the average mortality risk, . However, as approaches zero, the ratio becomes large for fixed , which increases the value of the inequality index. Thus a numerical problem may give a false impression that ELM inequality is increasing even as ELM rates tend towards zero.

The second problem is that probabilities are bounded by the interval, unlike income which is defined on the real line. For income inequality, for a set of incomes , and constant , for all measures (1) – (5) , income inequality measures remain unchanged if we multiply every income by . Scale-invariance, which is sensible for income data does not readily translate to mortality risk. The probability scale imposes serious constraints on as the distribution of also needs to be in the interval; at best must be less than 1 over the maximum of the s; for probabilities that cover the full range , only choices of are acceptable.

The third problem with inequality measures from the income inequality literature is that the inequality indices are not consistent with basic intuition about justice and health. Inequality measures on probability distributions should lead to the same conclusion whether we are measuring probability of mortality,

, or probability of survival, . A country with high inequality in infant mortality has equally high inequality in infant survival. Thus we assert a symmetry property in the evaluation of inequality on probability distributions

  • Symmetry: Suppose and are two probability distributions on . Then comparing inequality between and and comparing inequality between and should produce the same conclusion.

We use simulated data from a series of beta distributions with decreasing means that resemble mortality risk distributions to illustrate how inequality measures behave. Table

1 presents results for four distributions, where is fixed at for all four distributions and decreases from to , and . As decreases, the mean

and standard deviation

of mortality risk decrease. The mean gives the same interpretation whether we look at survival or mortality: survival is increasing and mortality is decreasing top to bottom; we interpret this as inequality is also decreasing from top to bottom. The standard deviation is the same whether we look at mortality or survival and suggests that inequality is decreasing from top to bottom. The coefficient of variation gives opposite stories about inequality for mortality and survival as do the Gini and Theil inequality indices: increasing from top to bottom for mortality, but decreasing for survival.

Income inequality measures such as CV, Gini or Theil do not satisfy the symmetry property and therefore are not suited to assess inequality in probability distributions. Income distributions are fundamentally different from probability distributions. The idea of scale invariance is neither necessary nor appropriate for measuring inequality in mortality because probabilities do not scale. In contrast, the mean or variance appear to be possible measures of inequality for distributions of ELM risk.

We now look at other methodologies to measure and describe inequality in ELM risk.

3 Analyzing Inequality in Early Life Mortality

In this section we develop appropriate tools to study inequality in mortality data. We begin with definitions and notation. Consider two populations we wish to compare, a reference population , and a comparison population . Each population has a distribution of mortality risk. The distributions are induced by a distribution on covariate space and transformations from space to probability space. In practice both and will need to be estimated which we discuss in section 5, but we take them to be known for the current discussion. Let be a random draw from ; then, for a logistic regression model, functions and

would be inverse logit functions

of or where and are regression coefficients for the reference and comparison populations. Populations and may differ in the distributions and of the covariates and in the values and of the regression covariates that multiply the covariates.

In comparing to , we will make interpretable adjustments to to make it be more similar to so as to understand how and why and differ.

3.1 Location-Scale Adjusted Distributions

It is useful to summarize differences between distributions based on a few summary measures. If and were continuous distributions on the real line, we might recenter and rescale so that the resulting distribution has the same mean and variance as . We create a new distribution, that represents a counterfactual or synthetic population. Then we could summarize the remaining differences between and

by a one number summary such as a Kullback-Leibler divergence or other divergence between distributions, or we could plot

and and inspect the differences graphically. General location-scale shifts do not work particularly well for distributions with bounded support, though with restrictions they can be useful. Let us assume that and enjoy full support on . Let be a location scale adjusted version of with . While could be negative, that flips the distribution around so that large probabilities become small and small probabilities become large, which violates the sense of manipulating one mortality distribution to be more like a second mortality distribution. Thus we need , , for to be a distribution contained in .

As mortality distributions are generally skewed with a mode close to zero and a long right tail, we prefer to restrict

and . Assuming the mean of is greater than the mean of , we can take . This leaves with support in the range . If we think of as the mortality risk in a country at an earlier time point and as mortality risk at a later time, then choosing is not a problem, as, in the current era, mean ELM risk is generally decreasing over time. If mortality risk increased, then we would manipulate instead of . Medians could certainly be used in place of means as well and we have used both means and medians in our work.

Thus summarizes the differences between the distributions and . To fully understand the differences between and , we would still need to summarize the differences between and .

3.2 Decomposition Methods

When comparing two distributions, decomposition methods are used to disentangle the effects of differences in coefficients versus from effects of differences in the distribution of the covariates (Kitagawa 1955; Oaxaca 1973). For example, mother’s age at the birth of an infant is an important covariate for estimating mortality risk as births from younger and older mothers have higher mortality risk. For two populations the relative importance of maternal age may differ and the distribution of maternal age can differ for different populations.

A useful adjustment is to take the distribution of covariates for either population or and the regression coefficient for the other population. Thus the distribution could be constructed as drawing at random from covariate population

, then multiplying by coefficient vector

from population and taking the inverse logit to construct adjusted distribution . A conditional version for sampling is also possible. Suppose that the density of in population is . We can construct adjusted probability distribution by drawing from a combination covariate density , then multiply by and taking the inverse logit. This constructs a population of probabilities based on population except that the covariate distribution has been adjusted to look conditionally like covariates from population .

The distributions that we have described here and in the previous subsection are called adjusted or counterfactual distributions.

3.3 Comparing Distributions

We now consider methods to quantify the differences between and by comparing to and also to . Let be a comparison (divergence, distance) operator between distributions that satisfies the triangle inequality

(6)

The Oaxaca decomposition (Oaxaca 1973), where is the absolute value of the relative difference in means, follows the triangle inequality when where is the mean of the adjusted distribution. Similarly, , the absolute difference in means also follows the triangle inequality.

3.4 Numerical Summaries for Comparing Distributions

It is often useful to quantify how much one distribution differs from another by a one number summary. Traditional approaches have summarized the distributions first with a one-number summary of the distribution and then compared the summaries. Measures such as the mean, variance, other moments, or quantiles can be used to compare and summarize distributions and we are not opposed to these methods, but we wish to consider other methods as well. We can measure how similar two distributions are by using divergence measures, provided the distributions share the same support. A commonly used measure is the Kullback-Leibler divergence,

(7)

where the densities and correspond to the distributions and . The divergence can be interpreted as the expected information for discriminating from based on a single observation from . Another useful measure is the norm

(8)

which quantifies how much probability mass needs to be moved so that one distribution becomes identical to the other one (Weiss 1995). The norm (8) is between 0 and 1, with 0 meaning the distributions are the same and 1 meaning the two distributions and do not share support.

4 Using Covariates to Explain Mortality Risk

Variation in mortality risk can often be explained by covariates. By analyzing subpopulations identified by levels of a categorical covariate, we split a large population into several subpopulations. Rather than being concerned with how one subpopulation compares to another subpopulation when we have a set of subpopulations, we are more concerned with how distinct the subpopulations are and whether targeting one or anther subpopulation for intervention might be useful for reducing overall ELM risk. However, we have found that even covariates that explain significant amounts of risk are not necessarily very useful in identifying subgroups to target so as to alleviate high risk ELM, a situation we illustrate in section 6.

4.1 ANOVA Methods

Consider a categorical covariate that identifies groups such as wealth quintiles or maternal age groups in our population. If most of the variability in risk occurs between groups, then knowing the mean risk for each group identified by is highly informative about inequality in the population. In contrast, if most of the variability in risk occurs within groups, then comparing mean risk between groups does not provide much information on risk variation.

Variation in risk can be expressed as the between group variance plus the weighted sum of the within-group risk variances. The law of total variance produces the decomposition

(9)

where is the average within group variance and is the between-group variance of the group means. We fit Analysis Of Variance (ANOVA) models using mortality risk as outcome and group membership as predictor and use as a measure of how much of the total variance in ELM risk can be explained by membership in a particular group.

As an example, in section 6 we run separate ANOVAs for covariates and for births from a given year in India. We consider whether the might be increasing or decreasing over the years, to identify whether inequality across covariate categories might be increasing or decreasing with time.

It is not necessary that the ANOVA model be as complex as the Bayesian hierarchical model used to model the data. We consider the ANOVA as a summary measure of the posterior, not a model on its own right.

4.2 Time Trends

We are particularly interested in time trends in ELM and how group comparisons evolve over time in India. We use year to define subpopulations, and then use additional covariates such as income quintiles to further refine the population of births into sub-sub-populations. We use ANOVA to quantify the within-year inequality within and across income groups and then assess the trend in this summary over time. Among other additional comparisons, we use divergence measures to compare the distribution of mortality in the baseline year against the distribution of mortality risk in each of the following years. We can construct synthetic populations holding the distribution of some key covariate constant to evaluate its effect on mortality risk time trend. For example, we can estimate the distribution of mortality risk over time if the distribution of maternal age was fixed over time.

5 Accounting for Uncertainty When Evaluating Inequality

We estimate risk for subjects with a Bayesian hierarchical logistic random effects regression models fit using Markov chain Monte Carlo (MCMC). For each iteration of the MCMC , we have a point estimate of for all births and we can now proceed with our program to evaluate ELM risk inequality as sketched in sections 3 and 4. For example, we can use the s and each birth’s wealth quintile to calculate an . However, this calculation is for only a single iteration of the MCMC. Thus for all iterations we calculate an producing a posterior distribution for . This uncertainty can be used in comparing various s for different covariates and thus determine which covariates do the best at distinguishing ELM risk.

As our sample is discrete, when we wish to calculate the divergence between two subpopulations of s, we use a kernel density smoother to first smooth out the distributions before calculating a divergence. Then, as with ANOVA s, we can calculate distributions for divergences.

In our graphs and numerical summaries we present point estimates and 95% pointwise credible intervals calculated from the multiple MCMC samples.

6 Modeling of Early Life Mortality Risk in India

Our data on early life mortality in India comes from the Demographic and Health Surveys, (DHS) http://www.measuredhs.com/. We use data from two DHS surveys to construct a restrospective panel from 1975 to 1997. A third wave covering more recent years is available but does not include district level information which we use in our models, so we were unable to make use of the third wave data. We analyze births to mothers aged 15–35 from 1975 through 1998 to reduce truncation and censoring. We analyze a total of 408,706 births from 141,999 mothers in 3,806 sampling clusters taken from 443 districts and 26 states.

6.1 Model Specification and Estimation of Mortality Risk

We fit a hierarchical Bayesian logistic regression model to estimate each infant ’s mortality risk using covariates, time, and time varying covariate effects.

Let index the infants, nested in mothers, nested in sampling clusters , nested in districts , nested in states . Year of birth is indexed by with for births from 1975 up to for births from 1997. Let

be the binary response variable, whether infant

with covariate vector born from mother , in sampling cluster , in district , in state , and in year was alive at the age of one or not . We specify random intercepts for mother and cluster and bivariate random intercepts and time slopes at the district and state level. Let . Our Bayesian hierarchical logistic regression is

(10)
(11)

where is the unobserved probability of mortality for infant , is a vector of unknown coefficients corresponding to the covariates in , is the mother random effect with variance , is the sampling cluster random effect with variance , and are district random intercepts and slopes with prior covariance matrix , and and are state random intercepts and slopes with prior covariance matrix . Define and . The random effects priors are , , , and .

Covariates at the child level are birth order, birth year, gender, maternal age, with household level covariates of religion, caste, wealth quintile, residence (rural or urban), and maternal education. We use splines to capture non-linearities in the time trend and investigated whether covariates have time-varying effects, though these analyses are not shown in this paper. We include main effects and all two-way interactions between covariates. The effects for maternal age, birth order, and birth year are modeled using b-splines.

The intercept is given a , prior, all main effects are given standard normal priors, and all two-way interactions are given priors. The web appendix gives the complete model specification.

While interest in individual probabilities is standard in hierarchical logistic regressions, we are not really interested in individual probabilities. Rather, we are interested in the entire collection of probabilities , simultaneously as the key quantity of interest from the model. This set of probabilities are used as inputs to our inequality calculations. We generate samples from the posterior distribution of the ELM risks, , and where indexes the MCMC samples.

6.2 Analysis of Inequality

Disparities in mortality risk are not well-captured by looking at national averages of mortality rates. For example, the infant mortality rate (IMR) in India was 12% in 1975 and 6% in 1995, both calculated as unadjusted means from our data. While this is a remarkable decline, these numbers do not quantify important aspects of the distribution of mortality risk as estimated by our model. To illustrate, figure 1

displays three kernel density estimates: one representing the distribution of mortality risk in India for 1975, which is the less peaked, longer right tailed density, and one for 1995, which has the higher mode and smaller right tail and a third density mostly hidden next to the 1995 density which we discuss in a moment. The shaded areas display 95% pointwise credible regions; we generated

kernel density estimates from MCMC samples and calculate the .025 and .975 percentiles at each point along the axis. The modal mortality risk was 2% in 1975 and 1% in 1997, which means that the actual mortality risk most infants experienced in both years were quite different from the IMR. In 1975 26% of infants have mortality risk higher than the 12% 1975 IMR while in 1995 only 11% of infants have mortality risk higher than the 1975 IMR of 12%. The uncertainty in the kernel density estimates shows that the distribution of ELM risk from our model is well determined in both years, and thus we are comfortable in asserting that the mortality risk distribution is stochastically much smaller in 1995 than in 1975. The third density plotted in Figure 1 is almost indistinguishable from that of 1997 and was constructed by taking the probabilities in 1975 and multiplying by the ratio of the 1997 median to the median for 1975. The resulting adjusted 1975 density is virtually indistinguishable from the 1997 distribution. Summary statistics from figure 1 are given in table 2.

Looking at the median of by wealth quintile we find that the mortality rate by quintile is: lowest quintile .08; second .07; middle .05; fourth .04; highest .03, which shows that the mortality rate is almost 3 times higher than the poorest group compared to the richest. However, these calculations ignore within-group variability. Figure 2 shows box plots of posterior means of mortality risk for every infant in our data, by wealth quintile, where denotes the entire data set. By contrast with the means by quintile, figure 2 shows that the distribution of mortality risk is highly variable within each wealth quintile. For example, 10% of births from the two lowest quintiles have mortality risk higher than 12% and 30% of births from other wealth quintiles have higher mortality risk that the median mortality risk among the poor. This suggests that summaries such as means hide useful information and that quantifying individual mortality risk is necessary.

We quantify how much of the changes in ELM over time can be explained by the multiplicative adjustment. We use 1975 as the comparison year to compare against each subsequent year . In figure 3 we use KL (left) and (right) divergence measures to compare differences in the distributions year versus 1975, with (lower dotted curve) and without (upper solid curve) the multiplicative median adjustment. Bands are pointwise 95% posterior intervals. The differences between the baseline year and the other years are close to zero after the adjustment and do not grow larger with time. The overall distribution of mortality risk in India grows increasingly different from the baseline distribution in 1975, yet these changes can mostly be explained by a parsimonious multiplicative shift.

Figure 4 analyses the effect of the change in maternal age distribution over time on the distribution of mortality risk. The left panel plots histograms of maternal age in 1975 (pink) and 1995 (grey). While there is a great deal of overlap in the age densities, maternal age in 1995 has fewer younger mothers as compared to 1975. The right-hand panel displays the distribution of ELM risk for 1975 and 1995 and an adjusted version of 1975 where we have adjusted the maternal age distribution to look like that in 1995. Adjusting for maternal age in 1975 changes the adjusted 1975 distribution slightly towards smaller values, but not far enough. Table 3 presents Kullback-Leibler and divergences quantifying the differences between the adjusted 1975 distributions of infant mortality and the 1995 (unadjusted) distribution after adjustments for the various covariates in our sample with 95% posterior intervals in parentheses. Single covariate adjustments alone do not explain the changes in distribution of mortality risk from 1975 to 1995.

Figure 5 shows the extent to which variability in mortality occurs within covariate categories and doesn’t change over time. We look at the for caste, religion, states, maternal education, wealth, residence, and districts over time. For all covariates and all times, the vast majority of variation in inequality in infant mortality is within categories and not between categories. For religious group and for caste, the covariate contains almost no information on the variability of inequality: for these groups, almost 100% of the variation is within category. These patterns are consistent over time. This suggests that socioeconomic groups described by a single covariate are much more heterogeneous than previously thought and that infants within a single sub-group defined by levels of wealth or other covariates have very different mortality risks. This also confirms the notion that comparisons between the means of groups defined by levels of a categorical demographic variables ignores substantial within-group variability in mortality risk due to the high levels of heterogeneity. This finding highlights the importance of measuring inequality within as well as between groups of births. This is also an important contribution to the literature on India and it has been overlooked by previous research.

7 Discussion and Conclusions

While in this paper we have looked at ELM, our results extend to mortality data in general, such as adult mortality. Because mortality risk is a probability, it is fundamentally different from income and our results suggest that measures from income inequality literature are inappropriate for mortality.

Our findings have implications for measures of inequality in mortality and shed light on how to design summary measures to quantify inequality in mortality. Currently inequality in early-life mortality uses measures from the income inequality literature or calculates average differences between large groups of births, such as between countries or between income groups within-countries. Our results show that these analyses are incomplete, and possibly misleading. For example, several studies have suggested that some Sustainable Development Goals (SDG) were not achieved due to high levels of inequality (NIMS et al. 2012; Houweling and Kunst 2010; Gwatkin 2005). UN General Assembly Resolution 68/261, which highlights Sustainable Development Indicators as a key measurement for measuring progress in reducing early-life mortality, recognizes this fact by recommending that health indicators should be disaggregated, where relevant, by subpopulations (Economic and Social Council: Statistical Commision 2016). Our approaches identify within-group variability and thus we can identify inequalities that are missed when one only looks at average levels.

Our methods have broad applicability to other health outcomes, including those that are not defined in the probability scale, such as life expectancy or malnutrition. Our methods are particularly useful when scientists suspect that within-group variability can be substantial or when researchers are interested in other aspects of the distributional differences between two populations besides mean differences. For example, our methods can be used to determine if the changes over time in height among children is driven by relatively slow growth of certain high-risk children versus faltering of the entire population.

Although we use a Bayesian approach in this paper, our methods are potentially compatible with a frequentist approach. If a researcher can fit a frequentist model and simulate predictions for mortality risk, for example using bootstrap methods, our methods can be applied to the bootstrap predictions. One notable advantage of the Bayesian approach is that is makes inference easier. For example, the popular LASSO Tibshirani (1996)

can do variable shrinkage and selection, but does not naturally provide standard errors for coefficient estimates and so also won’t provide uncertainty measurements for probabilities which are functions of the coefficient estimates. By contrast, in a Bayesian model, we can easily calculate the probability that one sub-population is more unequal than another sub-population by counting the proportion of MCMC samples in which one sub-population’s measure of inequality is higher than another. The frequentist approach can depend on being able to fit a complex model to bootstrap samples, something that can fail, for example in attempting to bootstrap a logistic regression with a small fraction of cases or where a variance component gets set to zero when using maximum likelihood.

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Apendix: Priors

We use gamma priors on the precision parameters. For the mother random effects,

Similarly, for clusters,

For districts,

For states,

All fixed effects are given mean zero normal priors. The intercept term has variance 9, all other main effects have variance 1, and two-way interactions have variance 0.5. In total, there are

Different combinations of covariates. The categorical covariates can have differing numbers of levels, so this gives a total of regression parameters to be estimated.