1. if you don't understand why you would use a control group which does not wear a mask to compare it with the group which did... I really can't help you. That is just a common sense understanding of statistics.... you either have a feel for how to test a hypothesis or you don't. In this case you want to have a large enough group spending enough time outside the home to see if the group wearing the mask gets infected at a statistically significant lower rate than a similar group which did not wear a mask.
2. Now you are bullshitting your ass off... just like GWB... try reading the study.
They even did a post hoc analysis of exactly what you wished for.
In a per protocol analysis that excluded participants in the mask group who reported nonadherence (7%), SARS-CoV-2 infection occurred in 40 participants (1.8%) in the mask group and 53 (2.1%) in the control group (between-group difference, −0.4 percentage point [CI, −1.2 to 0.5 percentage point];
P = 0.40) (OR, 0.84 [CI, 0.55 to 1.26];
P = 0.40).
Supplement Figure 2 provides results of the prespecified subgroup analyses of the primary composite end point. No statistically significant interactions were identified.
In the preplanned sensitivity analysis, those who had a positive result on an antibody test at 1 month but had not provided antibody results at baseline were considered to have had positive results at baseline (
n = 18)—that is, they were excluded from the analysis. In this analysis, the primary outcome occurred in 33 participants (1.4%) in the face mask group and 44 (1.8%) in the control group (between-group difference, −0.4 percentage point [CI, −1.1 to 0.4 percentage point];
P = 0.22) (OR, 0.77 [CI, 0.49 to 1.22];
P = 0.26).
Three post hoc (not preplanned) analyses were done. In the first, which included only participants reporting wearing face masks “exactly as instructed,” infection (the primary outcome) occurred in 22 participants (2.0%) in the face mask group and 53 (2.1%) in the control group (between-group difference, −0.2 percentage point [CI, −1.3 to 0.9 percentage point];
P = 0.82) (OR, 0.93 [CI, 0.56 to 1.54];
P = 0.78). The second post hoc analysis excluded participants who did not provide antibody test results at baseline; infection occurred in 33 participants (1.7%) in the face mask group and 44 (2.1%) in the control group (between-group difference, −0.4 percentage point [CI, −1.4 to 0.4 percentage point];
P = 0.33) (OR, 0.80 [CI, 0.51 to 1.27];
P = 0.35). In the third post hoc analysis, which investigated constellations of patient characteristics, we did not find a subgroup where face masks were effective at conventional levels of statistical significance (data not shown).
A total of 52 participants in the mask group and 39 control participants reported COVID-19 in their household. Of these, 2 participants in the face mask group and 1 in the control group developed SARS-CoV-2 infection, suggesting that the source of most observed infections was outside the home. Reported symptoms did not differ between groups during the study period (
Supplement Table 3).
I don’t follow your logic. If you’re trying to determine the effectiveness of wearing masks to protect you from contracting Covid, why would the group not wearing masks be your control? We know the rate of transmission from exposure will be high there already.
What this study is good for is looking at the effectiveness of wearing masks to prevent contraction, not spread. So, what is needed is a reliable control group that wears masks with high frequency. The group in this study was more random and self reporting- that data is not reliable, but interesting nonetheless.
I’d like to say that we wear masks to stop the spread, not the contraction. It’s an important distinction. Any benefit of wearing a mask to lessen contraction would be a bonus.
