Is sigma here standard deviation of 49 people that are randomly selected or whole population. I am also confused where sigma and sigma by root’n’ appear, when to use both of them.

sigma = 25cm , standard deviation of the population.

It made me wonder too, iff mean of population is not available then how one could go about getting std deviation.

I think the video is more about predicting mean of population from mean of sample using confidence interval.

In the later videos, namely " Unknown Standard Deviation" you can see how confidence interval is computed when std deviation of population is not known.

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Hi @AnilBhat,

I agree with you. If the population mean is not known, then the population standard deviation shouldn’t be either.