Thanks for responding!
I think it’s laudable to investigate the basis for claims as you’ve done. It’s fair to say evidence appraisal and communication really is a specialist area in its own right, and outside our ares of expertise it’s common to make errors in doing so. And while we all like evidence confirms what we think, other biases may be at play. I think some people in effective altruism also put a high value on identifying and admitting mistakes, so we might also be quick to jump on a contrary assessment even if it has some errors of its own.
I think your broader point about communicating the areas and extent of uncertainty is important, but the solution to how we do that when communicating in different domains is not simple. For example, you can look at how NICE investigates the efficacy of clinical interventions. They have to distill 1000′s of pages of evidence into a decision, and even the ‘summary’ of that can be 100s of pages long. At the front of that will be an ‘executive summary’ which can’t possibly capture all the ares of uncertainty and imperfect evidence, but usually represents their best assessment because ultimately they have to make concrete recommendations.
Another approach is that seen in the Cochrane Systematic Reviews. These take a very careful approach to criticising the methodology of all studies included in their analysis. A running joke though its that every Cochrane review reaches the same conclusion: “More Evidence is Needed”. This is precise and careful, but often lacks any practical conclusion.
Re your 2 questions:
It’s $7.14 for 1 eye (in 2001) with 77% success, according to this source:
In Toby Ord’s essay he uses this to derive the “less than $20 per person” figure (7.14 *2 /(0.77) = $18.5 )
So that’s both eyes (in 2001 terms).
My main area of uncertainty on that figure is around number needed to treat. I’ve spoken to a colleague who is an ophthalmologist and has treated trichiasis in Ghana. Her response was “trachoma with trichiasis always causes blindness”. But in the absence of solid epidemiology to back it up, I think it’s wise to allow for NNT being higher than 1. I would be comfortable with saying that for about $100 we can prevent trachoma-induced blindness, in order to contrast that with things that we consider a reasonable buy in other contexts. (I haven’t assessed any orgs to know if there are orgs who do it for that little: they may for instance do surgeries on a wider range of conditions with varying DALYs gained per dollar spent).