# population standard deviations known, how

Hi there, I got stuck with a funny question which I cannot figure.

- Question #1: Where does the population standard deviation $15000 come from?

I read the answer to a similar question and it still does not make any sense to me

"How can you have a known population variance without having all the population data?" by Bon Crowder

https://learn.365datascience.com/question/how-can-you-have-a-known-population-variance-without-having-all-the-population-data/

Indeed, when stdev.p and stdev.s in excel calculated from the given sample the results are very different from the given $15k (11285.48, 11478.41 respectively).

I hope somebody can help with this one.

Additionally I find a following question in the above mentioned thread very intriguing.

- Question #2: "So in practice how to actually know if population variance is (considered) known or unknown ?" by matthieu bonjour.

Can anyone help with this one as well please?

Thanks

Lu

Hi, Lu. After reading your question, I got stuck too, but I think I've figured it out. The population standard deviation is given here without any calculations because this lesson is about getting the confidence interval from a population **with its variance known**. The file that is provided to us already has the standard deviation of $15,000, which implies that the variance is known. If we want to get the exact number of the known variance, we simply raise $15,000 to the second power.

As for the second question, I honestly don't know. Hopefully we will help with that one.

Cheers,

Julián