Thanks a lot for the comment. I do think that what your gesturing at makes sense: if I understand correctly you are saying that certain physical interventions can have more predictable effects that ‘biological’ ones because we have a decent idea of exactly how they work. In some cases this is definitely true: as an extreme example, we don’t need RCTs of aeroplane safety as we have a very good understanding of the physical processes and are able to model them well. If we have an airborne pathogen, it’s hardly necessary to run an RCT to see whether or not there is an effect of a stay at home order: there will be one.
In many of the example questions I gave though, I think the fact that there is a large behavioural component pushes us closer to the situation we have with drugs than to the aeroplane. For example, although it could be demonstrated in a laboratory which of mask or shield is actually more effective at blocking exhaled particles, it would be harder to capture the different effects that each has on how often you touch your face, how often it is removed, or other aspects of compliance. These will differ a lot between people, so you’d need to test it on a large group, and the social setting might influence behaviour. I don’t think that we can decompose the often important behavioural component of these interventions in the same way that we can the physical components.
That said, the air filtration question I posed might not have been well chosen. As you point out, it seems reasonable that we can get a good understanding of whether that is likely to be helpful by applying what we know about the filters and viral transmission. Of the questions I posed, RCTs are likely to be the least useful there and may not be useful at all.
However, I do have some thoughts on why an RCT could still be worthwhile. I’m not saying these because I disagree with your points; I’m just providing some possible counterargument.
Learning: by introducing the filters not in an RCT, you are basically doing an experiment but losing the opportunity to learn from it. Even if it has been decided that filters should be introduced in all schools/offices (or whatever unit), it won’t normally be possible to install all of those in parallel. So there is a time where some offices have the filter and some don’t. As long as you can randomise this, you can take advantage of the differences in time for implementation in something like a stepped wedge cluster randomised trial. The effect could be analysed on an ongoing basis in a Bayesian analysis such that if there are large effects they would be detected early in the experiment and implementation of the remaining filters can be accelerated. If you are doing something like this across several interventions, this would help with deciding which to prioritise.
Cost-benefit: There are ~ 137,000 schools in the US. I don’t know how much it costs to install and maintain filtration systems, but I imagine it is not negligible. There are a lot more schools globally. Doing an RCT comparing e.g. air filtration to opening the windows could save quite a bit of money if it turns out that filtration systems don’t provide additional benefit.
Implementation and interaction with behaviour: even assuming that they do work, do people use them? Maybe the filtration is noisy so teachers turn it off; maybe they simply forget to turn it on. In medicine, even with drugs that demonstrably improve the patient’s condition, adherence is (to me) surprisingly low. Perhaps large rooms where people tend to congregate most cannot be adequately filtered, maybe the filtration system gives people a sense of security so they congregate more.
Overall, I think the areas where trials would be most useful are those where we can expect relatively modest effects and where there is a larger behavioural component. The combination of modest effects, if better understood, might be quite important.
I like this perspective. I’ve never really understood why people find the repugnant conclusion repugnant!