Years lived with a disability

Years lived with a disability (YLD) due to each disease can be calculated on the basis of either the incidence or the prevalence of the disease. The initial GBD studies estimated YLD on the basis of the incidence of each disease. Thus, in the 1990 study for example, the YLD estimates measured the future loss of health resulting from disease episodes that began in 1990. One advantage of this approach is that it is consistent with that used for mortality: YLL measure the future loss of life resulting from deaths in a particular year.

The 2010 GBD study adopted the alternative approach and calculated YLD based on the prevalence of the impairments resulting from each disease in the year for which the estimates are made. This approach has the advantage that it assigns YLD to the ages at which they are lived, rather than to the age at which the disease episode that produced them began.

Using the prevalence based approach, the YLD associated with each disease and injury, i, in each age group, x, are calcuiated by multiplying the disability weight for each of the 220 health states, W_h, by the prevalence, p_h,i,x, of that health state as a sequela of that particular disease and the population at risk in the age group, N_x, and:

$$YL{D}_{i,x}=\sum _{h=i}^{220}{W}_h·p_{h,i,x}·N_x$$

The prevalence of most of the 220 health states is zero for any particular disease.

The distinction between incidence based and prevalence based estimates of YLD matters little for diseases characterised by acute episodes of impairment of short duration. It matters more for diseases that lead to long-lasting impairments.

If the incidence of a disease is changing, prevalence based estimates of YLD will reflect its past incidence, rather than its current incidence. Thus, the burden of disease from conditions that cause long-lasting disability but are in decline, such as tooth loss, is emphasised in the 2010 estimates compared with earlier GBD studies and the burden of long-lasting conditions such as diabetes that are causing increasing amounts of disability is given less weight.