The evolution of the disability-adjusted life year (DALY)

Highlights

• We review the methodological progression of the DALY, a summary health measure.

• Changes in the DALY components have improved the DALY metric.

• Component changes make it difficult to interpret, compare, and conduct DALY studies.

Abstract

The disability-adjusted life year (DALY) is a summary health measure that combines mortality and morbidity into a single measure as a way to estimate global disease burden and the effectiveness of health interventions. We review the methodological progression of the DALY, focusing on how the use of life expectancy estimates, disability weights, age weighting, and discounting has evolved since the first DALY reports were published in 1993. These changes have generally improved the metric but have made it difficult for researchers to interpret, compare, and conduct DALY studies.

Introduction

The disability-adjusted life year (DALY) made waves in the international development community when it was introduced in the 1993 World Development Report [1]. Previous estimates of global burden of disease generally focused on mortality rates, both because reducing fatalities was a top public health priority and because deaths are easy to count [2]. However, mortality data alone are not sufficient for painting a picture of the state of health in a community, nation, or region. As mortality rates began to level off in industrialized nations toward the late twentieth century, researchers from a variety of disciplinary perspectives created new health metrics that incorporated physical and psychological morbidity and disability in addition to mortality, including the quality-adjusted life year (QALY) [3] and the healthy year equivalent (HYE) [4]. The DALY, which estimates the gap between a population's health status and an “ideal” level of health and survival, emerged as a commonly used tool. Economists, epidemiologists, and policy experts, especially those who work on health issues in low- and middle-income countries, frequently use the DALY for population health assessments, priority setting, and program evaluation. By contrast, decision scientists, health economists, and policymakers in high-income countries more frequently use the QALY metric.

The conceptual framework for the DALY uses the term “disability” to refer to any acute or chronic illness that reduces physical or mental health status in the short-term or the long-term. “Disabilities” in DALY models include conditions such as quadriplegia, total blindness, and developmental disorders as well as infectious and parasitic diseases, nutritional deficiencies, maternal and perinatal conditions, a diversity of non-communicable and neuropsychiatric conditions, and injuries. DALYs aim to quantify at the population level the total years of life lost to premature death and the years of life lived with suboptimal health due to any condition that reduces functioning partially or fully for a short period of time or a long duration.

While the underlying conceptual model for the DALY remains unchanged, the DALY has been under continuous revision since it was first developed by World Health Organization and World Bank collaborators in 1993 [1]. Major changes to DALY estimation methods were made for the Global Burden of Disease (GBD) estimates for 1990 [5], [6], [7], [8], [9] and 2010 [10], [11], [12], [13], [14], [15], [16]. In the intervening years, updated GBD estimates were published annually from 1999 to 2004, [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27] and several major regional European studies were published in the 2000s [28], [29], [30], [31]. DALY methods have also been used for cost-effectiveness analysis in low- and middle-income countries [32], [33], [34], [35]. DALYs represented a major step forward for population health metrics [36]. However, as researchers have tweaked the equations used for estimating the DALY and have challenged some of the assumptions underlying these calculations, comparing DALY estimates across time has become difficult.

All versions of the DALY quantify the burden of disease by combining mortality and morbidity in a single metric. The basic equation for the DALY is the sum of a population's years of the life lost (YLL) to premature death and the years lived with disability (YLD):

$$DALY=YLL+YLD$$

The most basic equation for the YLLs lost in a population during a particular time period, such as one year, is:

$$YLL=N\times L$$

where N is the number of deaths in the population and L is the population's average remaining life expectancy, in years, at the age of death. The basic equation for YLDs in a population is:

$$YLD=\left(I\times L\right)\times W=P\times W$$

where I is the number of incident cases of a particular condition in the population, L is the average length (duration) of disability from a particular condition, P is the prevalence of the condition, and W is the disability weight associated with the condition.

However, neither YLLs nor YLDs can be directly measured. The YLL is dependent on the researcher's selection of the total years a member of the population is, on average, “expected” to live. The YLD depends on how disability weights are assigned for various health conditions or consequences. Additionally, some DALY models apply discounting and age weighting functions that generally apportion higher YLL and YLD values to current health problems and those that affect the young, and assign lower values to future health concerns and ones that primarily affect older adults. Thus, the number of DALYs estimated for a population may be vastly different depending on the assumptions made. Two research groups working with the same population data about births, deaths, incidence, and prevalence could arrive at very different sets of DALY estimates.

Most DALY estimates are derived from complex mathematical models that account for age distributions, population dynamics such as birth and age-specific death rates, and even socioeconomic strata. These more computationally intense approaches require more cumbersome equations. A discounted, age-weighted YLL can be calculated within a model by an equation such as this one [5]:

$$YLL=\frac{KC{e}^{ra}}{{\left(r+\beta \right)}^2}\left\{e^{-\left(r+\beta \right)\left(L+a\right)}\left[-\left(r+\beta \right)\left(L+a\right)-1\right]-e^{-\left(r+\beta \right)a}\left[-\left(r+\beta \right)a-1\right]\right\}+\frac{1-K}{r}\left(1-e^{-rL}\right)$$

where K is an age weighting value, C is a constant that ensures that the total number of DALYs worldwide remains the same with and without age weighting, a is the age at death, r is the discount rate, β is a constant that adjusts the shape of the age weighting curve, and L is the standard expectation of remaining years of life at the time of death at age a. The expanded equation for YLD is similarly unwieldy [5].

These models require assumptions about life expectancy, disability weights for numerous causes of reduced health status, discount rates, and age weighting. Each of these assumptions requires careful consideration when attempting to estimate the DALYs in a population or interpreting reports of DALYs. This paper summarizes the evolution of the DALY, focusing on how the use of these four particular components has changed over time. Knowledge of the history of the DALY is a necessary foundation for calculating, interpreting, and comparing DALY estimates.