The purpose is to measure the effect of a single variable, so you make sure to correct for all other variables. For example, to measure the effect of height you might compare white men only against white men, black women only against black women, etc.
In a study measuring the gender pay gap, they would be correcting for variables other than gender.
It also depends a lot on the shape and size of your face and head, so what works best for me might not work so well for you.
https://www.youtube.com/watch?v=TRCZ8Qnf0Z0
If you want to be more certain of the fit of your respirator, this video outlines in detail how to do a fit test using a solution made from commonly available saccharin sweeteners (make sure you use saccharin and not a different artificial sweetener, bitter fit testing solution can also easily be purchased online as an alternative).
The process itself isn't too difficult, and the required equipment (a device to aerosolize the solution and a hood to concentrate it in the air around your head) is easy to find and inexpensive.
IIRC N95s shaped like the Aura fit a larger variety of faces than rigid cup shaped ones like the one you posted. They're also usually more comfortable. It might be worth it for you to try some other respirators if the ones you have are painful in some situations.
Aaron Collins (known as the mask nerd) is a great resource for in depth information on masks if you want to know more.
blaming her for going to a popular dentist
@[email protected] you stand accused of being a dental elitist, cruelly judging those who frequent popular dentists. How do you plead?
Obviously the problem isn't the popularity of the dentist, the problem is the dentist not having any precautions in place and the appointment being in the middle of a large wave of infections.
Houthi Rebels
They are the government.
"Hitler is basically the German Trump"
universally reviled in the Middle East
He is literally a phophet in the Quran.
I'm also trying to fit a SodaStream in my budget if I can
Israeli company that treats Palestinians like shit on stolen land, should be boycotted.
The PFLP has boycotted the PLO for almost a decade, does operate in Gaza, and is participating in the current hostilities.
"Israel only accidentally commits genocide and ethnic cleansing. Hamas deliberately killed soldiers and settlers!"
It's actually worse, I think he's telling a black man that the police killings that led to BLM were "tragic inadvertent loss of life". 
Yes, that's what it means - if all other factors they list are equal, meaning if the only difference is height.