Uncertainty Arising from Epidemiological Estimates
Uncertainty in YLD estimates is mainly determined by the uncertainty in (a) epidemiological estimates for the prevalence and/or incidence of disability associated with specific causes or cause groups; and (b) disability weights arising from uncertainty in health state valuations and, in some cases, also in the disability severity distribution associated with a condition.
For a subset of the GBD causes, analysts carrying out reviews and analyses for the estimation of YLD also estimated levels of uncertainty in regional prevalences. These assessments took into account not only typical levels of measurement error in the input data sets, but also expert judgment on the degree of uncertainty arising from the lack of representativeness of the available data for each region. The resulting uncertainty ranges vary considerably across causes, ranging from relatively certain estimates for some causes such as polio, for which intensive surveillance systems are in place, to highly uncertain estimates for other causes such as osteoarthritis, where for some regions not a single usable dataset was found, and where for others the latest available data were decades old. The summary tables provided in chapter 3 for numbers of data sources used for YLD estimates by cause and region provide one indication of the relative uncertainty associated with YLD estimates for different causes.
For some causes, such as stroke and ischemic heart disease, YLD estimates were essentially derived from estimates of cause-specific mortality by means of models of regional variations in case fatality rates. In such cases, YLD uncertainty will be significantly higher than the uncertainty associated with cause-specific mortality estimates given the considerable uncertainty in case fatality rates for most low-and middle-income countries and in models used to infer the burden of nonfatal disease from mortality. YLD uncertainty will generally be greater than YLL uncertainty, and will also vary across causes according to both the typical uncertainty associated with the measurement of incidence or prevalence according to GBD case definitions and with the number and representativeness of available studies. For a subset of 16 major causes of YLD for which analysts estimated indicative uncertainty ranges, the typical uncertainty for regional prevalence estimates ranged from 10 percent to 90 percent, with a median value of 41 percent. Uncertainty ranges were generally higher for low- and middle-income countries than for high-income countries.