If you're talking to two people, one with a small cut and another with multiple sclerosis, everyone present will agree that having multiple sclerosis is much worse. If you offered the two of them some magic option that would restore exactly one of them to full health, they would probably be able to agree on who should get it. But in general, how should we compare across people to figure out whose situation is worse, who would benefit more from treatment and who, everything else being equal, should be treated first? This example was easy because the difference was nice and large, but what do we do in harder cases?

One is to ask people questions like "if there were a surgery that could restore you to full health (without improving your lifespan) but had a 20% chance of killing you, would you take it?" If they say "yes" then this indicates that this disability, for this person, is more than 20% as bad as being dead. Ask these "standard gamble" questions to a lot of people with a lot of disabilities, varying the percentages, and you could build up a list of how bad different ones are, all on a common scale.

This would useful for balancing projects against each other, figuring out what to focus on, and generally setting funding priorities. Unfortunately people are really bad at answering questions like this. Mostly we're just bad at thinking about percentages and chances of bad things happening, but you also wonder about the problems of asking someone with, say, "Schizophrenia: acute state" to answer this sort of question.

You could fix this by asking people about "time tradeoffs". For example, you could ask someone with a disability about whether they would take a medicine that would restore them to full health for a year even if it took two years off their life. Alternatively you can ask people, generally public health professionals, if given the choice between curing 1000 people with disability X and 2000 people with disability Y which one they would choose. These "person tradeoffs" get us out of needing to ask about probabilities, which means we can probably trust the numbers more, but we're stuck either with collecting data from people with the disabilities in question (hard work, maybe the disability affects mental function) or trusting that public health experts fully understand what it's like to have different disabilities (seems unlikely). And even if we did decide that we were only going to collect data from people actually affected by a disability, remember that in many cases they can't actually give the comparison we want because they haven't experienced both having and not having the disability (ex: blindness from birth).

The first Global Burden of Disease Report (pdf) attempted to collect these weights for a large number of different diseases. They got a panel of public health experts to come to Geneva and through discussion around "person tradeoffs" came to consensus first on weights for 22 "indicator diseases". Then they agreed on weights for the several hundred remaining diseases by comparing them to these anchor conditions.

You might worry that the final weights would be really strongly affected by the particular consensus the Geneva group happened to get for the 22 anchors, but they ran nine other attempts with different experts and the average of those attempts correlates pretty well with the Geneva results:

For the 2010 update to the Global Burden of Disease weights (pdf, also see the appendix pdf) they decided to take an entirely different approach. Instead of asking experts to figure out tradeoffs they asked lots of people in several countries (Indonesia, Peru, USA, Bangladesh, Tanzania, plus a 'global' internet survey) to do lots of comparisons where given two people they would say which one was healthier. This makes a lot of sense, asking lots of regular people, and the question is much simpler to answer. On the other hand, while I'm not sure experts are all that good at estimating how bad it is to have various disabilities I would expect regular people to be even worse at it. Still, there was at least pretty good correlation between countries:

But hold on: how did they turn a large number of responses where people said one disability was more or less healthy than another into weightings on a 0-1 scale where 0 is full health and 1 is death? It turns out that a quarter (n=4000) of the people who took the survey on the internet were also asked the "person tradeoff" style questions used in the 1990 version, which they called "population health equivalence questions". So they first determined an ordering from most to least healthy using their large quantity of comparison data, and then used the tradeoff data to map this ordering onto the "0=healthy 1=death" line.

This means that when we look at the cross-country correlations above we're only seeing their agreement on the relative ordering of conditions, not on the absolute differences. If people in Indonesia on average think that the worst disabilities are only 10% as bad as being dead while people in Peru think they're 90% as bad, this wouldn't keep them from having perfect correlation on a chart like this. Which is kind of a problem, because we need more than an ordering for prioritization.

Update 2014-09-17: See Toby and Owen's comments below; they actually did something more complex, involving treating "A is sometimes ranked above and sometimes below B" as an indicator that A and B are similarly harmful.

It turns out that this method of estimation actually gives pretty different results from the one used in the earlier version: [1]

Yes, there's a correlation, but it's pretty weak. And it's probably not just about the fifteen years between when most of the first estimates were made and when most of the second were; these aren't disabilities that are quickly changing. This indicates that what we're trying to measure is just not that well captured by the measurements we're making.

So, a summary. Getting good answers means asking people questions they're not good at thinking about, or that they are good at thinking about but don't have the right experience to be able to answer. It's not too surprising, then, that the answers you get via different methods don't agree very well. We do still need a rough way to say "benefit X to N people is better/worse than benefit Y to M people," but trying to do this in the general case doesn't seem to have worked out very well.

These disability weights are only one step in estimating $/DALY for various interventions, and the messiness here is in some ways much less than the messiness in the other steps. After reading about how these estimates came to be, I'm pretty glad GiveWell doesn't put much trust in them in figuring out which charities to recommend:

The resources that have already been invested in these cost-effectiveness estimates are significant. Yet in our view, the estimates are still far too simplified, sensitive, and esoteric to be relied upon. If such a high level of financial and (especially) human-capital investment leaves us this far from having reliable estimates, it may be time to rethink the goal.

All that said—if this sort of analysis were the only way to figure out how to allocate resources for maximal impact, we'd be advocating for more investment in cost-effectiveness analysis and we'd be determined to "get it right". But in our view, there are other ways of maximizing cost-effectiveness that can work better in this domain—in particular, making limited use of cost-effectiveness estimates while focusing on finding high-quality evidence.

This isn't to say we should never use disability weights; even if they were just made up by one guy on the spot (and they're better than that) this would probably still be better in some cases than refusing to make quantitative comparisons at all. Getting rough numbers like this is especially useful for avoiding scope insensitivity problems, where you might be comparing a large number of people with something minor against a small number with something major.

(I'd really like to look into the QALY numbers people use and how they get them. I believe the process is similar, but I'm not too sure.)

For curiosity, however, and with all that in mind, what are the actual numbers they found? Here are the 2010 weights:

0.756	Schizophrenia: acute state
0.707	Multiple sclerosis: severe
0.673	Spinal cord lesion at neck: untreated
0.657	Epilepsy: severe
0.655	Major depressive disorder: severe episode
0.641	Heroin and other opioid dependence
0.625	Traumatic brain injury: long-term consequences, severe, with or without treatment
0.606	Musculoskeletal problems: generalised, severe
0.576	Schizophrenia: residual state
0.573	End-stage renal disease: on dialysis
0.567	Stroke: long-term consequences, severe plus cognition problems
0.562	Disfigurement: level 3, with itch or pain
0.549	Parkinson's disease: severe
0.549	Alcohol use disorder: severe
0.547	AIDS: not receiving antiretroviral treatment
0.539	Stroke: long-term consequences, severe
0.523	Anxiety disorders: severe
0.519	Terminal phase: without medication (for cancers, end-stage kidney or liver disease)
0.508	Terminal phase: with medication (for cancers, end-stage kidney or liver disease)
0.494	Amputation of both legs: long term, without treatment
0.492	Rectovaginal fistula
0.480	Bipolar disorder: manic episode
0.484	Cancer: metastatic
0.440	Spinal cord lesion below neck: untreated
0.445	Multiple sclerosis: moderate
0.438	Dementia: severe
0.438	Burns of >=20% total surface area or >=10% total surface area if head or neck, or hands or wrist involved: long term, without treatment
0.433	Headache: migraine
0.420	Epilepsy: untreated
0.425	Motor plus cognitive impairments: severe
0.422	Acute myocardial infarction: days 1-2
0.406	Major depressive disorder: moderate episode
0.390	Fracture of pelvis: short term
0.399	Tuberculosis: with HIV infection
0.398	Disfigurement: level 3
0.388	Fracture of neck of femur: long term, without treatment
0.388	Alcohol use disorder: moderate
0.383	COPD and other chronic respiratory diseases: severe
0.377	Motor impairment: severe
0.376	Cocaine dependence
0.374	Low back pain: chronic, with leg pain
0.373	Lower airway burns: with or without treatment
0.369	Spinal cord lesion at neck: treated
0.366	Low back pain: chronic, without leg pain
0.359	Amputation of both arms: long term, without treatment
0.353	Amphetamine dependence
0.352	Severe chest injury: short term, with or without treatment
0.346	Dementia: moderate
0.338	Vesicovaginal fistula
0.333	Burns of >=20% total surface area: short term, with or without treatment
0.331	Tuberculosis: without HIV infection
0.329	Cannabis dependence
0.326	Abdominopelvic problem: severe
0.323	Gastric bleeding
0.322	Low back pain: acute, with leg pain
0.319	Epilepsy: treated, with recent seizures
0.312	Stroke: long-term consequences, moderate plus cognition problems
0.308	Fracture of neck of femur: short term, with or without treatment
0.200	Iodine-deficiency goitre
0.294	Cancer: diagnosis and primary therapy
0.293	Gout: acute
0.292	Musculoskeletal problems: generalised, moderate
0.288	Drowning and non-fatal submersion: short or long term, with or without treatment
0.286	Neck pain: chronic, severe
0.281	Diarrhoea: severe
0.269	Low back pain: acute, without leg pain
0.263	Parkinson's disease: moderate
0.259	Autism
0.259	Alcohol use disorder: mild
0.254	Infectious disease: post-acute consequences (fatigue, emotional lability, insomnia)
0.236	Conduct disorder
0.235	Severe traumatic brain injury: short term, with or without treatment
0.225	Crohn's disease or ulcerative colitis
0.224	Traumatic brain injury: long-term consequences, moderate, with or without treatment
0.223	Bulimia nervosa
0.223	Anorexia nervosa
0.221	Neck pain: acute, severe
0.221	Motor plus cognitive impairments: moderate
0.221	HIV: symptomatic, pre-AIDS
0.210	Infectious disease: acute episode, severe
0.202	Diarrhoea: moderate
0.198	Multiple sclerosis: mild
0.195	Distance vision blindness
0.194	Fracture of pelvis: long term
0.194	Decompensated cirrhosis of the liver
0.192	Fracture other than neck of femur: short term, with or without treatment
0.192	COPD and other chronic respiratory diseases: moderate
0.191	Distance vision: severe impairment
0.187	Disfigurement: level 2, with itch or pain
0.186	Heart failure: severe
0.177	Fetal alcohol syndrome: severe
0.173	Fracture of face bone: short or long term, with or without treatment
0.171	Poisoning: short term, with or without treatment
0.171	Musculoskeletal problems: legs, severe
0.167	Angina pectoris: severe
0.164	Anaemia: severe
0.164	Amputation of one leg: long term, without treatment
0.150	Fracture of sternum or fracture of one or two ribs: short term, with or without treatment
0.159	Major depressive disorder: mild episode
0.157	Intellectual disability: profound
0.149	Anxiety disorders: moderate
0.145	Crush injury: short or long term, with or without treatment
0.145	Cardiac conduction disorders and cardiac dysrhythmias
0.142	Urinary incontinence
0.130	Amputation of one arm: long term, with or without treatment
0.136	Injured nerves: long term
0.132	Fracture of vertebral column: short or long term, with or without treatment
0.132	Asthma: uncontrolled
0.129	Dislocation of knee: long term, with or without treatment
0.127	Severe wasting
0.127	Burns of >=20% total surface area or >=10% total surface area if head or neck, or hands or wrist involved: long term, with treatment
0.126	Intellectual disability: severe
0.123	Abdominopelvic problem: moderate
0.110	Lymphatic filariasis: symptomatic
0.110	Asperger's syndrome
0.114	Musculoskeletal problems: arms, moderate
0.106	Traumatic brain injury: long-term consequences, minor, with or without treatment
0.105	Chronic kidney disease (stageIV)
0.101	Neck pain: chronic, mild
0.099	Diabetic neuropathy
0.097	Epididymo-orchitis
0.096	Burns of <20% total surface area without lower airway burns: short term, with or without treatment
0.092	Hearing loss: complete, with ringing
0.080	Intellectual disability: moderate
0.080	Dislocation of shoulder: long term, with or without treatment
0.088	Hearing loss: profound, with ringing
0.087	Fracture of patella, tibia or fibula, or ankle: short term,with or without treatment
0.086	Stoma
0.082	Dementia: mild
0.070	Heart failure: moderate
0.070	Fracture of patella, tibia or fibula, or ankle: long term, with or without treatment
0.070	Benign prostatic hypertrophy: symptomatic
0.079	Musculoskeletal problems: legs, moderate
0.079	Injury to eyes: short term
0.076	Stroke: long-term consequences, moderate
0.076	Motor impairment: moderate
0.073	Fracture of skull: short or long term, with or without treatment
0.072	Severe toothloss
0.072	Fracture of neck of femur: long term, with treatment
0.072	Epilepsy: treated, seizure free
0.072	Disfigurement: level 2
0.066	Angina pectoris: moderate
0.065	Injured nerves: short term
0.065	Hearing loss: severe, with ringing
0.065	Fracture of radius or ulna: short term, with or without treatment
0.061	Herpes zoster
0.061	Diarrhoea: mild
0.050	Fracture of radius or ulna: long term, without treatment
0.058	Hearing loss: moderate, with ringing
0.058	Anaemia: moderate
0.057	Fetal alcohol syndrome: moderate
0.056	Severe chest injury: long term, with or without treatment
0.056	Acute myocardial infarction: days 3-28
0.055	Kwashiorkor
0.054	Speech problems
0.054	Motor plus cognitive impairments: mild
0.054	Generic uncomplicated disease: anxiety about diagnosis
0.053	Infectious disease: acute episode, moderate
0.053	HIV/AIDS: receiving antiretroviral treatment
0.053	Fracture other than neck of femur: long term, without treatment
0.053	Fracture of clavicle, scapula, or humerus: short or long term, with or without treatment
0.051	Amputation of both legs: long term, with treatment
0.040	Neck pain: acute, mild
0.040	Headache: tension-type
0.049	Attention-deficit hyperactivity disorder
0.047	Spinal cord lesion below neck: treated
0.044	Amputation of both arms: long term, with treatment
0.030	Intestinal nematode infections: symptomatic
0.030	Anxiety disorders: mild
0.030	Amputation of finger(s), excluding thumb: long term, with treatment
0.038	Mastectomy
0.038	Hearing loss: mild, with ringing
0.037	Heart failure: mild
0.037	Angina pectoris: mild
0.035	Bipolar disorder: residual state
0.033	Hearing loss: complete
0.033	Fracture of foot bones: short term, with or without treatment
0.033	Fracture of foot bones: long term, without treatment
0.033	Distance vision: moderate impairment
0.032	Hearing loss: severe
0.031	Intellectual disability: mild
0.031	Hearing loss: profound
0.031	Generic uncomplicated disease: lly controlled
0.025	Fracture of hand: short term, with or without treatment
0.024	Musculoskeletal problems: arms, mild
0.023	Musculoskeletal problems: legs, mild
0.023	Hearing loss: moderate
0.023	Diabetic foot
0.021	Stroke: long-term consequences, mild
0.021	Amputation of one leg: long term, with treatment
0.019	Impotence
0.018	Ear pain
0.018	Burns of <20% total surface area or <10% total surface area if head or neck, or hands or wrist involved: long term, with or without treatment
0.017	Fetal alcohol syndrome: mild
0.017	Dislocation of hip: long term, with or without treatment
0.016	Fracture of hand: long term, without treatment
0.016	Claudication
0.015	COPD and other chronic respiratory diseases: mild
0.013	Near vision impairment
0.013	Disfigurement: level 1
0.013	Amputation of thumb: long term
0.012	Motor impairment: mild
0.012	Dental caries:symptomatic
0.012	Abdominopelvic problem: mild
0.011	Parkinson's disease: mild
0.011	Infertility: primary
0.009	Other injuries of muscle and tendon (includes sprains, strains, and dislocations other than shoulder, knee, or hip)
0.009	Asthma: controlled
0.008	Periodontitis
0.008	Amputation of toe
0.006	Infertility: secondary
0.005	Open wound: short term, with or without treatment
0.005	Infectious disease: acute episode, mild
0.005	Hearing loss: mild
0.005	Anaemia: mild
0.004	Distance vision: mild impairment
0.003	Fractures: treated, long term

(gbds-2010--disability-weights.tsv)

[1] Technically these are the results from the 2004 update to the 1990 version, but when you look at where their estimates come from (pdf) you see that most just say they're kept unchanged from the 1990 version.