# Confidence levels

#### Mpp

##### New Member
Trying to find confidence levels given CI, std deviation and sample size. Everything I look up explains how to find the intervals. What am I missing.

#### hlsmith

##### Not a robit
What are "confidence levels"? Can you provide a lengthier description of the problem and its context?

#### Mpp

##### New Member
The percent if confidence. For example I know the confidence intervals are (41.6,44) and the random sample size is 100. Standard deviation is 6. I need to find what level of confidence this meets. I’ve tried working “backwards” but must be missing something

#### Mpp

##### New Member
I know the answer. I just don’t understand how they got it. 94.5%

#### ondansetron

##### TS Contributor
Write out the formula and plug in all the variables you have values for in this problem. Which variable is the unknown? Hint: if you know the interval end points are equidistant from the point estimate, you can just solve one side. Otherwise solve each.

What do you get at this point?

#### Mpp

##### New Member
Mean +- Z • 3/5. Mean = 42.8. I know this is wrong but I am just stuck

#### hlsmith

##### Not a robit
So you have everything but the standard normal value, right? Well if you are getting close to say 1.96, it's probably just a rounding error, even if you have the dataset, the mean, std, or zvalue - they can all be rounded. Post all of the parts and we will tell you what we get.

#### Mpp

##### New Member
Based on a random sample of 100, you have confidence intervals for the true mean yield of (41.6,44) and the yield has std dev of 6 what was the level of confidence used to obtain this interval

#### ondansetron

##### TS Contributor
Again, write out the formula for the upper bound and separately for the lower bound. Solve for the unknown and tell us what you get. From there you can be helped to find the confidence coefficient. (You probably only need to do it once in this case, but if there interval isn't symmetric you need to do both.)

They're just asking if you can manipulate the formula and look up the probability associated with a critical value.

#### hlsmith

##### Not a robit
I get 95.44% Confidence Interval.

I can provide more details if needed, see if you can use this to get their interval values.

#### abhay9318

##### New Member
You have c.i ,s.d. and sample size
since, sample size is 100 we will use large sample theory
let us suppose that the statistics is mean
then
lower limit =mean - z(alpha)*s.e

where confidence level = 100(1-alpha)%
you just have to select alpha
to get the corressponding confidence level
you can go to excel and use norminv func for that enter norminv(alpha,0,1) it will return for one-tail.