Sensitivity of Burden of Disease and Injury Results to Variations in Key Parameter Values
This section examines the sensitivity of the DALY estimates for the global burden of disease in 2001 to alternative assumptions about the discount rate and age weighting. As discussed in chapter 3, the DALY measures the future stream of healthy years of life lost due to each incident case of disease or injury. It is thus an incidence-based measure rather than a prevalence-based measure. The GBD study applied a 3 percent time discount rate to years of life lost in the future to estimate the net present value of years of life lost. With this discount rate, a year of healthy life gained in 10 years' time is valued at 24 percent less than one gained now (note that the standard DALY uses an instantaneous 3 percent discount rate, which results in an annual discount factor that is slightly higher).
Table 5.3 summarizes the effects of varying the discount rate and age weights. Changes in the discount rate and age weights have little effect on the proportion of the burden in males and females. However, changes in the discount rate have an important effect on the proportion of the burden due to nonfatal outcomes (YLD), on the age distribution of the burden, and on the distribution of the burden by broad cause group. When the discount rate is set to zero, the proportion of burden due to YLD is just over a quarter of the total burden. When the discount rate is set to 3 percent, then 36 to 38 percent of the burden is due to YLD, depending on whether age weights are also applied.
[Table .]
Similarly, a nonzero discount rate significantly reduces the importance of the burden of disease or injury in children. This effect is more pronounced in low- and middle-income countries, where children bear a disproportionately large share of the total burden (figure 5.5). Because of the differences in the cause structure of the disease burden by age, these effects also influence the overall distribution of DALYs by broad cause group for low- and middle-income countries. In contrast, for high-income countries, while some changes in the age distribution of the burden are apparent, the choice of discounting (and age weights) has relatively little influence on the broad cause group breakdown of the total burden of disease (figure 5.5).
[Figure
5.5]
The effects of introducing nonuniform age weights are generally much smaller than the effects of introducing nonzero discounting. A comparison of the discounted DALYs with and without age weighting in table 5.3 shows that the main effect is on the age distribution of the disease burden. For both high-income and low- and middle-income countries, age weights reduce the importance of the share of the burden borne by older people. In low- and middle-income countries, people aged 60 years and older suffer 21 percent of the total burden of disease and injury. This declines to 13 percent when nonuniform age weights are used. As shown in the second part of figure 5.5, the effects of discounting and age weighting on the age structure of the burden of disease largely offset each other for older ages, so that for DALYs(0,0) and DALYs(3,1) the share of the burden for those aged 60 years and older is quite similar. Overall, the importance of Group I conditions (communicable diseases, maternal and perinatal conditions, and nutritional deficiencies) is also slightly enhanced by age weighting and that of Group II conditions (noncommunicable diseases) is reduced. The effects on Group III (injuries) are relatively minor.
Figure 5.6 compares the rank order of causes contributing to the global burden of disease measured using DALYs(3,1) and DALYs(3,0). The introduction of nonuniform age weights has the most impact on neuropsychiatric disorders, such as bipolar disorder, panic disorder, and obsessive-compulsive disorder, whose prevalence is greatest in younger and middle-aged people. Age-weighted DALYs give less importance to causes whose burden falls predominantly on older ages.
[Figure
5.6]
Figure 5.7 compares ranks for causes measured using undiscounted DALYs(0,0) and discounted DALYs(3,0), both with uniform age weights (K = 0). A zero discount rate gives greater importance to causes with a larger burden at younger ages, such as whooping cough (pertussis) and meningitis, and lower importance to causes predominantly affecting older ages. However, the different choices of discount rates and age weights do not cause any large changes in the rank ordering of diseases and injuries, which is to a large degree anchored in absolute differences in the burden arising from large differences in prevalence and mortality levels across causes.
[Figure
5.7]
Table 5.4 compares DALYs(3,0) with DALYs(3,1) and DALYs(0,0) in more detail according to the second level of cause disaggregation within a group. These more detailed results confirm the major conclusions outlined earlier on the impacts of discounting and age weighting. DALYs(0,0) give greater weight to perinatal conditions (the International Classification of Diseases [ICD] category of conditions arising in the perinatal period) and respiratory infections, which primarily affect young children, than either of the discounted forms of DALYs. In contrast, the age-weighted DALYs(3,1) give more weight than DALYs(3,0) to causes that predominantly affect younger adult ages, such as neuropsychiatric conditions and injuries. DALYs(3,0) give greater weight than either DALYs(3,1) or DALYs(0,0) to causes that predominantly affect older people, such as cardiovascular diseases and cancers.
[Table .]
Figure 5.8 summarizes the effects of changing the discount rate and age weighting on the global rankings for the top 20 causes of the burden of disease in 2001. The left-hand column shows the rankings for causes measured using DALYs(3,0) as used for the DCPP. The middle column is for DALYs(3,1), as used by WHO to present the GBD analysis. The principal difference is that the use of DALYs(3,0) results in relatively greater importance being placed on chronic diseases of middle and older ages, such as ischemic heart disease and stroke, and somewhat lesser on HIV/AIDS, road traffic accidents, congenital anomalies, and other disorders affecting children and younger adults. Undiscounted DALYs, shown in the right-hand column, give proportionately greater importance to conditions affecting children, such as malaria and measles.
[Figure
5.8]
