# Thread: standard deviation

1. ## standard deviation

What do you do when you are told to find the critical value and you are given conference interval and n but the population standard deviation is unknown?

I need help with:
98%, n=17, population standard deviation is unknown
90%,n=199, population standard deviation is unknown, population appears to be normal.

2. Originally Posted by fairymonkey
What do you do when you are told to find the critical value and you are given conference interval and n but the population standard deviation is unknown?

I need help with:
98%, n=17, population standard deviation is unknown
90%,n=199, population standard deviation is unknown, population appears to be normal.
If you knew the SD how would you find the confidence interval?

So the confidence interval end points lie at mean+/-k*SD, but you are given these end points and the means to find k, do that then solve for the SD.

RonL

3. I don't understand what you just said. I don't know how to use your formula without knowing what the SD is or the mean.

4. Originally Posted by fairymonkey
What do you do when you are told to find the critical value and you are given conference interval and n but the population standard deviation is unknown?

I need help with:
98%, n=17, population standard deviation is unknown
90%,n=199, population standard deviation is unknown, population appears to be normal.
All you need is to decide what test-statistic is to be used then look up the critical value in a table. In your problem I'd be considering the t-statistic for the appropriate degrees of freedom .......

Eg. In the first one, $\alpha = 0.02$ so you need the value of $t_{0.01}$ for 16 degrees of freedom etc.