[ummonk]

Agree with the title, but not the contents. The study in question is actually an example of a huge effect size (10% reduction in cases just from instructing villages they should wear masks is amazing) possibly hampered by poor statistical significance (as the blog post outlines).

[robocat]

Without knowing how many people were wearing masks, you can’t say the much about the 10% figure.

You get approximately[1] the same outcome if:

(a) masks are 100% effective but only 10% wear them, and

(b) masks are 10% effective and 100% wear them.

Is this study showing (a) or (b)?

Let us assume (b) masks only help by 10% and R0 is 2 without masks. If exponential transmission is occurring then in ~11.5 days you have the same number infected with masks as in 10 days without masks.

Either way the study has ended up with a 10% figure, and that figure gets misunderstood or intentionally misrepresented. If you want to argue for the effectiveness of masks against those that don’t wish to wear them, then personally I think it is a terrible study to argue with because 10% sounds shitty.

[1] Actual numbers depends on a heap of other things, but just assume those figures are right for the sake of making things easy to understand.

Disclaimer: I wear a mask during Level 2 lockdown in the South Island of New Zealand, and mask wearing has no partisan meaning here AFAIK.

[derbOac]

Also, the study, IIRC, found greater social distancing in the mask conditions, which leads to other possible explanations.

I wear a mask all the time and am happy to but I agree this study, while solid in some respects, is not exactly overwhelming in making a compelling argument for masks.

[vannevar]

It should also be noted that the positive effect rose to 35% for people over the age of 60, who make up the overwhelming majority of serious Covid-19 cases. The omission of this important fact from the article leads me to question the motivation of the author.