Disability-Adjusted Life Year

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Introduction & definition

DALY = Disability Adjusted Life Year = the sum of years of potential life lost due to premature death, and the years of productive life lost due to disability.

DALYs are used to measure the combined quantity and quality of life of a population.

The WHO website summarises the DALY thus:
"One DALY can be thought of as one lost year of 'healthy' life. The sum of these DALYs across the population, or the burden of disease, can be thought of as a measurement of the gap between the current health status and an ideal health situation where the entire population lives to an advanced age, free of disease and disability."

ie. One DALY is equivalent to the loss of a healthy life year.

Calculation of DALY

This measurement is a combination of 2 measurements (shown below):
DALY = YLL + YLD
ie. Disability Adjusted Life Year [DALY] is equal to the sum of the Years of Life Lost [YLL] due to premature mortality in the population and the Years Lost due to Disability [YLD} for people living with the health condition.

YLL

YLL = Years of Life Lost

This consists of the number of deaths multiplied by the standard life expectancy at the age at which death occurs.

The formula for this is: YLL = N x L

N = number of deaths and L = standard life expectancy at age of death (in years)

YLD

YLD = Years Living with a Disability

There are two methods of calculating the YLD.

1. The number of cases, in a particular time period, multiplied by the average duration of the disease combined with a weight factor which represents the severity of the disease on a scale from 0 (perfect health) to 1 (dead).

The formula is: YLD = I x DW x L

I = number of incident cases

DW = disability weight

L - average duration of he case until remission or death (in years)

2. Known as the Prevalence YLD:

The formula is: YLD = P = DW

P = number of prevalent cases

DW = disability weight

Function/Uses of DALYs

DALYs are used to indicate the impact of disease on a population.

DALYs for a number of diseases can be added together to indicate the total burde of disease in a country.

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