In the last decade, a non-financial approach to valuing human life has been derived, where loss of wellbeing and premature mortality are measured in DALYs. This approach was developed by the WHO, the World Bank and Harvard University for a study that provided a comprehensive assessment of mortality and disability from diseases, injuries and risk factors in 1990, projected to 2020 (Murray and Lopez, 1996). Methods and data sources are detailed further in Murray et al (2001) and the WHO continually revisits these estimates.

A DALY of 0 represents a year of perfect health, while a DALY of 1 represents a year dead. Other health states are attributed values between 0 and 1 as assessed by experts on the basis of published quality of life data for various health states. For example, the disability weight of 0.02 for mild vision loss can be interpreted as a 2% reduction in a person’s quality of life relative to perfect health. The DALY approach has been successful in avoiding the subjectivity of individual valuations and overcomes the problem of comparability between individuals and between nations, although some nations have subsequently adopted variations in weighting systems (e.g. age weighting for older people). This study considers the value of a life year (disability weighting) as equal throughout a person’s lifespan (i.e. independent of age). Under the DALY framework, the total burden of disease for an individual with a condition is the sum of the mortality and morbidity components associated with that condition over time, including the years of healthy life lost due to disability (YLDs), and the years of healthy life lost due to premature death (YLLs). Incorporating time preference for health (and thus discounting), this is represented by:

Let Dw sub i, t be the disability weight for individual i at time t YEARS into the condition. The total disability suffered by the individual is the Dw open bracket i,t close bracket divided by open bracket 1+r close bracket raised to the power of t-a and summed from t = a to t = a + L

r is the annual discount rate

where Dw is the disability weight of the condition experienced by individual i, L is the residual life expectancy of the individual at age a, and t represents individual years within that life expectancy.

The total burden of disease from a condition on society is the sum of the DALYs for all individuals with the condition. In this study the total DALY burdens for people with mild VI, moderate VI, and blindness are calculated for the years 2010, 2015 and 2020.

5.1.1 Disability weights for vision impairment and blindness

Any weighting exercise for use in burden of disease analysis or economic evaluation should measure preferences for clearly defined and relevant health states. Two key sources of disability weights for VI have been identified: the WHO Global Burden of Disease (GBD) study (Murray and Lopez, 1996) and a Netherlands study (Stouthard et al, 1997).

The 1990 GBD study asked participants in weighting exercises to make a composite judgement on the severity distribution of various health conditions and the preference for time spent in each severity level for those conditions (Lopez et al, 2006). This was to a large extent necessitated by the lack of population information on the severity distribution of most conditions at the global and regional level. The WHO Global Burden of Disease and Risk Factors study has used the GBD study weights to estimate the DALY burden from various causes and risk factors in different regions of the world (Lopez et al, 2006).
GBD study weights for VI and blindness vary by cause of low vision or blindness, according to the disability weights for treated and untreated VI, and the likelihood of treatment. The GBD definitions of VI match the standard WHO levels of VA:

• Low vision (3/60 ≤ VA < 6/18);
• 0.282 if untreated;
• 0.227 if treated;
• Blindness (VA < 3/60);
• 0.6 if untreated; and
• 0.488 if treated (cataract and diabetes mellitus retinopathy only).

These VA definitions differ from this study, where blindness is defined as a VA less than 6/60. Furthermore, the GBD study did not estimate disability weights for people with a VA less than 6/12 and equal to or greater than 6/18, defined as mild VI in this study.

Netherlands researchers measured disability weights for 53 diseases of public health importance using a methodology consistent with the GBD study (Mathers et al, 1999; Stouthard et al, 1997). The Netherlands study used more specific disease stages and severity levels so that judgements were not required on the distribution of stages or severities in the population. In addition, the study defined each disease stage by the associated average levels of disability, handicap, mental wellbeing, pain and cognitive impairment using a modified version of the EuroQol health status instrument (Mathers et al, 1999). The Netherlands study weights are:

• 0.02 for mild vision loss (some difficulty reading newspaper, no difficulty recognising faces at 4 meters);
• 0.17 for moderate vision loss (great difficulty reading newspaper, some difficulty recognising faces at 4 meters); and
• 0.43 for severe vision loss (unable to read newspaper or recognise faces at 4 meters).

Netherlands study weights have been used in Australian burden of disease studies undertaken by the government (Begg et al, 2007; Mathers et al, 1999), and by Access Economics in burden of VI studies for five countries and worldwide (Access Economics, 2004; 2006; 2008a; 2008b; 2009; 2010). As with the GBD study weights, the Netherlands study health states do not completely concord with this study, but are used here rather than the GBD weights for three reasons:

• consistency with previous Access Economics studies;
• the Netherlands weights cover three severities of VI, consistent with this study; and
• GBD weights are more likely to overestimate the DALY burden given that low vision and blindness are defined as more severe than in this study (a VA cut-off of 3/60 rather than 6/60). Conversely, the Netherlands definitions may, particularly for mild and moderate VI, include less severe cases of vision loss leading to conservative DALY estimates.

The Netherlands study weights for mild vision loss, moderate vision loss, and blindness were used for mild VI, moderate VI, and blindness, respectively, in this study.

5.1.2 Willingness to pay and the value of a statistical life year

Because DALYs are not a financial metric they are not directly comparable with monetary costs and benefits associated with a specific condition. In an economic evaluation of a public program, a monetary conversion of the loss in healthy life is typically used to ascertain the cost of a condition and, in turn, the net benefit or cost of a health intervention. This allows benefit/cost ratios to be calculated so comparisons can be made across all types of programs, not just those associated with changes in health.

In general there are two ways to estimate the value of a change in the stock of health capital using survey techniques. The first is to directly measure the willingness to pay (WTP) for a change in health status using a choice based approach, such as contingent valuation or discrete choice methods.

The alternative is to model the WTP for a year of healthy life using the value of a statistical life (VSL) currently used in the public arena. The VSL is generally derived from the WTP of individuals to avoid small changes in the risk of various health states, often including death. As this is arguably a similar context to deriving WTP for changes to morbidity, VSL estimates can be applied to summary health measures such as quality adjusted life years (QALYs) and DALYs.

Since no estimates for the VSL in the ROI could be identified, this study employs the VSL estimated for the UK by Mason et al (2009). This VSL was used by Access Economics (2009) to estimate the burden of sight loss in the UK. Mason et al (2009) derived the VSL from a UK Department for Transport analysis where the public were asked about their WTP for a reduction in death from road safety improvements using a contingent valuation/standard gamble approach (UK Department for Transport, 2007). The VSL was estimated to be £1.43 million in 2005 prices. Adjusting for quality of life, discounting at a rate of 1.5% (the recommended rate of pure time preference by the UK Treasury), and adjusting for the value of consumption forgone due to death, the value of a QALY estimated to be £70,896 in 2005 prices by Mason et al (2009). A QALY is equivalent to a year of perfect health, and the monetary value of a QALY is therefore the value of a statistical life year (VSLY).

As part of their study, Mason et al (2009) conducted an international literature review on the estimated VSLY for different countries. The UK VSLY estimated by Mason et al (2009) was higher than two Swedish studies (Johannesen and Meltzer, 1998; Person and Hjelmgren, 2003) but lower than the median VSLY in an international review of studies calculating the implicit value of a QALY (Hirth et al, 2000). An earlier Australian study (Abelson, 2003) reported a lower VSLY than the UK estimate. Therefore, the UK figure is within the bounds of other international studies.

The UK VSLY reported by Mason et al (2009) was inflated to 2010 prices (£81,318) using the estimated inflation increase from 2005 to 2010 of 14.7% (UK National Statistics, 2010) and then converted to euros at the latest available exchange rate of €1 = ₤0.86 (ECB, 2011) to attain an estimate of €94,794. This estimate of the current VSLY in the UK has been used as a proxy for the value of a DALY in the ROI in this study.

It should be noted that Access Economics’ Australian cost of VI study (Access Economics, 2010a) used a VSLY of AUD$161,751 in 2009, which inflates to AUD$166,604 in 2010. Converting this to VSLY to euros at the latest available exchange rate of €1 = $A1.44 (ECB, 2011) gives a VSLY of €115,512, which is one-fifth higher than the UK VLSY used in this study.