The equation for the Disability Adjusted Life Year is simple:

DALY = Years Lived with Disability (YLD) + Years of Life Lost (YLL)

For YLL, a standardized table assigns years of life lost to an early death. So for example if a newborn dies, they have lost 86 years of life (based on the highest life expectancy, found in Japan).

YLD is calculated by adding a disability weight adjustment to any years lived with a disease or injury. So a year of full health adds no DALYs, a year of death adds 1 DALY, and a year lived with say, metastatic cancer adds 0.45 DALYs.

In other words, a year lived with cancer is counted as half as healthy as a year of full health.

Where does that figure come from? You, or someone like you.

IHME have surveyed 60,890 people in nine countries with questions similar to a game you may have played as a kid: would you rather be deaf or blind? But in their version of the game, there’s no mention of the condition. The survey questions describe the symptoms of a particular condition in lay terms, so that the stigma associated with that condition doesn’t affect the outcome.

Try out part of the survey. Click on ‘Reveal Conditions’ to see what the results tell us.

Using the data generated from the survey, IHME calculated a disability weight adjustment for over two hundred health states.

Disability weight adjustments are a number between 0 (perfect health) and 1 (death). They are the first step towards calculating the Global Burden of Disease (GBD), a population-level snapshot of the world’s health.

But can a survey really capture all the nuances of health? In particular it seems challenging to condense the experience of a disease into a description the length of a tweet, and the wording is clearly going to impact the weight adjustments.

IHME go to great lengths to highlight uncertainty in the disability weights study. For example, they were surprised how highly illicit drug use disorders ranked, and suggest that perhaps they didn’t get the wording of the symptoms quite right.

But there is a danger these details are lost in further studies. It seems unlikely that a policy paper based on a cost-effective study based on the global burden of disease study based on the disability weights study will discuss concerns over the wording of the largely unknown survey that underpins it all.

Nonetheless, in some cases results are so emphatic the uncertainty intervals become insignificant.