Disability Weights

September 17th, 2014
ea, 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:

0.756 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

[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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