Calculating the confidence interval for the mean value from a sample The central limit theorem states that the means of many different samples with n>30 from a population that follows an arbitrary distribution are normally distributed.
If you now have the information from only one sample, how do you calculate the confidence interval, which states that with 95% the true mean value of the population is in this interval?
The normal distribution of the mean values is not available, since only one sample was taken, with only one mean value $\overline{x}$ and standard deviation s.
The estimated value for the mean is simply $\overline{x}$, the estimated value for the standard deviation is simply s, that is trivial. But we now want to have an interval with a confidence of, say, 95%.
Let's imagine the normal distribution that results from many samples. We don't have the results of many samples, but let's imagine this normal distribution. In the middle is $\mu_\overline{x}$ and to the left and right of it are the standard deviations, which of course we don't know either. Now we think of a range as being drawn in, which contains 95% of all sample means. Exactly this range we can also form around our determined sample mean $\overline{x}$ so that we can state: with 95% certainty the mean of all sample means (i.e. the population mean) lies in this area.
So what we need to calculate now is the 95% interval of the normal distribution, because we can then "put" this range over the sample mean x to get the limits of the interval. This is:
Lower limit = $\overline{x}$ - 1.96 * $\sigma_\overline{x}$
Upper limit = $\overline{x}$ + 1.96 * $\sigma_\overline{x}$
Whereas $\sigma_\overline{x}$ denotes the standard deviation of the normal distribution.
How do you get the $\sigma_\overline{x}$? I have read that it can be calculated by dividing the standard deviation s of the population by $\sqrt{n}$ . But since we don't have the sigma of the population, we divide the standard deviation s (of the sample) by $\sqrt{n}$. But why can we just use s here? Why can we use just the standard deviation of the sample to calculate the interval length of the normal distribution? A too high standard deviation of the sample makes the interval too large, a too low standard deviation makes the interval too small.

We could apply S there since this estimate of sigma (std err of population) is unbiased and consistent estimate.
