 The simple idea is complicated by two adjustments, however. Typical burden of disease studies weigh an impairment differently depending on the age of the person whose functional capacity is impaired by the disease or disability. Blindness, say, has a greater impact on the burden of disease if it occurs at age 20 than if it occurs at very young or older ages (Prüss-Üstün et al. 2003). Moreover, if the man who has the accident now can be expected to live with the disease for 30 years, future years of disability are discounted by a factor. The further into the future a disability occurs, the less it contributes to the disease burden (ibid.).