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Disability Weights

September 17th, 2014
giving, health

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:

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

[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.

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