29 Nov 2022

Posted on:

08 Aug 2022

4

# 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

Posted on:

29 Nov 2022

1

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