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See, that’s what the app is perfect for.

Sounds perfect Wahhhh, I don’t wanna
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neoliberalism-nightly asked:

Can you give me a or two good article on why people should use QUALY for utiltarian calculations.

slatestarscratchpad answered:

What level are we talking about here? Do you want to know what QALYs are, or why they might be better than DALYs or some other complicated thing?

slatestarscratchpad:

neoliberalism-nightly:

What are DALYs?

So: QALYs are quality adjusted life years. They’re years of your life, but penalized for them being supposedly bad. For example, they surveyed some people and found that they would really dislike being blind, such that a year of their life blind was only half as good as a year of their life sighted. So a year blind is worth 0.5 QALYs.

DALYs are the same, except that they also incorporate age. Since being old sucks (citation needed), maybe a year of your life at 80 is only worth half as much as a year of your life at 20 (this is an example, I don’t know the actual weighting function).

The reason these are useful is to compare the costs and benefits of different interventions. For example, which should we do - cure blindness or cure malaria?In order to do this calculation, we need to know the utility weights of blindness vs. death in ways that are convertible with each other.

QALYs are useful because we assume (maybe falsely) that everyone values a QALY the same amount - certainly this is more true than in the case of money, where a poor person really wants $1000 but a rich person doesn’t care about it at all.

Suppose that it cost $50 to cure one case of blindness in a child who would go on to live 50 more years. And it costs $1000 to cure one case of otherwise-fatal malaria in a child who would go on to live 50 more years. So we calculate:

Since blindness costs half a QALY per year, preventing 50 years of blindness produces 25 QALYs. Since that costs $50, that intervention costs $2 per QALY.

Since death costs one QALY per year, preventing 50 years of death produces 50 QALYs. Since that costs $1000, that intervention costs $20/QALY.

Therefore, the blindness intervention is more effective charity than the malaria intervention.

Also, the First World medical system has decreed that it will aim to do all treatments that cost less than $60,000/QALY, so this gives us a good measure of medical cost-effectiveness. If somebody invents a surgery that will make you live one year longer, but it costs $1 million, it’s not worth Medicare’s resources - in the very absolute sense that it will take away money from the Medicare pot that could be used more efficiently on someone else. Once you agree to use all money as efficiently as possible, $60,000/QALY is the number that falls out of how much funding the system has and how much various medical interventions cost.

DALYs are probably better if (like me) you think that people probably value some parts of their lives more than others, but they’re harder to calculate with.

Scott, there is an additional major difference between DALYs and QALYs which you did not cover.

QALYs are defined in terms of an individual’s utility from birth (or from the time a decision is made) until death.  During that time, he may experience some person-time that to him is worth 50% of what it would have been worth if he was in full health.  At the end of his life, if he was in full health until he was 75 and then lived 10 years at low quality, you can meaningfully say that he lived 80 quality-adjusted life years. 

DALYs are defined in terms of an individual’s <i>lost</i> utility from onset of disease until they would have died had they not had the disease. This is a counterfactual concept. Since we can never know when a person would have died if he had not had the disease, this makes the concept itself kind of metaphysical at the individual level.

Of course, if you are going to estimate the effect of an intervention in terms of QALYs, you are also going to have to make assumptions about what would have happened if you cured the disease. The difference between the two approaches is whether the counterfactual theory is incorporated in the definition of the construct, or if it is something that you have to keep in mind when you do the statistics. 

The philosophy behind QALYs is that for any individual, that person has one potential QALY outcome under treatment, and another potential QALY outcome under no treatment.  You as the scientist can only observe one of the two potential outcomes, but with the right statistics you may be able to compare the two potential outcomes on average in the population he was part of.

The philosophy behind DALYs is that for any individual, you can somehow observe the period of time between their death and the time their death would have occurred if the world was different.

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I once took a very formal, mathematical course on decision theory for public health. When we got to the theory of how to measure utility, the course focused almost exclusively on QALYs instead of DALYs, in part because QALYs are conceptually cleaner and because QALYs make much more sense in the Von Neumann Morgenstern axiom system.  

In VNM,  individuals choose between two lotteries. For instance, you may offer a person a choice between lottery 1  (which gives you x% chance of living the rest of your life at full health, otherwise you die immediately) and lottery 2 (which gives you 100% chance of living the rest of your life at your current health state).  This corresponds to the idea of QALYs, where the quality weight is defined in terms of the percentage probability of dying that would make a person indifferent between the two lotteries.

In contrast, you can’t really describe the DALY idea of “years lost to disease” in terms of a choice between lotteries..

My sense is that there is a group of people around Christopher Murray who push for DALYs, and pretty much everyone else ignores them because QALYs are simply defined in a better way. 

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