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Confidence interval of mean

Both images are 2 consecutive pages of my notes . In this theory , i was told to use z-distribution when the sample size is large ( more than 30) and the standard deviation of the population , σ is known) . However , in the 2nd image , i was told to replace σ with s ( sample standard deviation) . Which is correct ?

I think the first theory of using z-distribution when the sample size is large ( more than 30) and the standard deviation of the population , σ is known) is correct , and the* replace σ with s ( sample standard deviation) . is wrong ***. Am i right ?**

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Re: Confidence interval of mean

Quote:

Originally Posted by

**xl5899** Both images are 2 consecutive pages of my notes . In this theory , i was told to use z-distribution when the sample size is large ( more than 30) and the standard deviation of the population , σ is known) . However , in the 2nd image , i was told to replace σ with s ( sample standard deviation) . Which is correct ?

I think the first theory of using z-distribution when the sample size is large ( more than 30) and the standard deviation of the population , σ is known) is correct , and the* replace σ with s ( sample standard deviation) . is wrong ***. Am i right ?**

Attachment 37906Attachment 37907

It is always better to use $\displaystyle \sigma$ instead of $\displaystyle s$; however, in practice you will often be given a large set of data and will only be able to compute $\displaystyle s$. When your data are large enough, you may approximate $\displaystyle \sigma$ with $\displaystyle s$.

Bottom line: Use $\displaystyle \sigma$ if you have it, but don't hesitate to use $\displaystyle s$ if your data are large as an approximation to $\displaystyle \sigma$.

I hope this clears things up for you.

Best,

Andy

Re: Confidence interval of mean

Quote:

Originally Posted by

**abender** It is always better to use $\displaystyle \sigma$ instead of $\displaystyle s$; however, in practice you will often be given a large set of data and will only be able to compute $\displaystyle s$. When your data are large enough, you may approximate $\displaystyle \sigma$ with $\displaystyle s$.

Bottom line: Use $\displaystyle \sigma$ if you have it, but don't hesitate to use $\displaystyle s$ if your data are large as an approximation to $\displaystyle \sigma$.

I hope this clears things up for you.

Best,

Andy

Let me add one more bit to this. The Z-test **assumes** the standard deviation(s) is(are) known.