There are some 368 million DAILY active users on youtube.
It is making claims based on a dataset of roughly 0.0003% of the population of users.
It's a GUESS. A bad one at that, since the people who install that extension are absolutely not representative of the general youtube user.
If we expand it out to the 2.28 BILLION monthly active userbase... the data from the 14k users is basically meaningless.
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Think of it this way - if you were seconds in the day, those extension users are 25 seconds. if I were to try to measure any sort of meaningful data in a day by using 25 seconds of data, I would likely be horribly, horribly wrong.
Ex: My water company billed me and it's bullshit, I've been carefully tracking usage data for 30 seconds after I wake up every day, and I never measure any usage! Why are they billing me?
Holy cow, I measured our water usage today and we used a whole gallon over the 25 seconds I measured!!! We're blowing through nearly 3000 gallons a day!
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Both are horribly, horribly wrong estimates. A sample size that small is not very valuable.