This course will help prep you to earn the CompTIA Healthcare IT Technician certification showing that you have the knowledge and skills needed to succeed in installing, managing, and troubleshooting IT systems in medical and clinical settings.

Hi,

If I supply say 50 values in col A, how would I calculate the confidence interval and number of samples required to achieve a defined confidence level of say 95%?

So if my samples only returned a confidence level of say 46% it would say I need an additional 100 samples to achieve 95%?

Cheers

If I supply say 50 values in col A, how would I calculate the confidence interval and number of samples required to achieve a defined confidence level of say 95%?

So if my samples only returned a confidence level of say 46% it would say I need an additional 100 samples to achieve 95%?

Cheers

Experts Exchange Solution brought to you by

Enjoy your complimentary solution view.

Get every solution instantly with Premium.
Start your 7-day free trial.

I wear a lot of hats...

"The solutions and answers provided on Experts Exchange have been extremely helpful to me over the last few years. I wear a lot of hats - Developer, Database Administrator, Help Desk, etc., so I know a lot of things but not a lot about one thing. Experts Exchange gives me answers from people who do know a lot about one thing, in a easy to use platform." -Todd S.

Experts Exchange Solution brought to you by

Your issues matter to us.

Facing a tech roadblock? Get the help and guidance you need from experienced professionals who care. Ask your question anytime, anywhere, with no hassle.

Start your 7-day free trialthe key part of the post by matthewspatrick is

"Taking a bigger sample is not going to increase your confidence level. Rather, having a larger sample makes the width of the confidence interval narrower."

He has given you a good introduction to sampling and a link to more infromation which should help you understand and answer the problem.

Thanks for that I have added to my sheet, but not making much sense, using the deviation difference between the mean and 2points before and after the mean I get around 95%+ of values in that range, and yet the proper confidence calculation to the right of the sheet only shows about 6-7% of values fall between the mean and confidence range?

Sample attached

SamplePX.xlsm

That is not how you use a confidence interval. As I indicated above, the common usage for a confidence interval is to show how the precision of a sample mean when it is used to estimate a population mean.

You

In a normally distributed population, it is true that approximately 95% of the members will be within two standard deviations of the mean. However:

That is **not the same thing** as saying "95% will be within the confidence interval for the sample mean"

In looking at how your data are being generated, your source data are not themselves normally distributed

Indeed, when I built my sample file posted in http:#a38822401, I used =ROUND(NORM.INV(RAND(),100

But now we're getting kind of far afield. I think your original question has been answered :)

To generate random data like that:

=MEDIAN(5,18,NORM.INV(RAND

The problem comes where you use something like 11.5 for the mean and 7.5 for the standard deviation. If you force all the values to be between 5 and 18, your sample will not be normally distributed, because you are truncating the tails.

The calculation of Sample size to give an accuracy (resolution) with a 95% CI is, I belive, what the essence of the question is about.

To determine sample size you need to know:

Process Deviation s

95% CI = x +/- r = x+/- 1.96s/ sqrrt(n)

95% CI ~ x+/- 2s/sqrrt(n)

Resolution r = 2s/sqrrt(n)

So for example if we wish to estimate cycle time within +/- one minute (r=1)

And we estimate the standard deviation to be five minutes (s = 5)

n = ((2*5)/1)^2 = 100

So if you know the reolution you are trying to acheive you can calculate the nescessary sample size to acheive a 95% confidence.

I hope this makes sense, I had to get my old SixSigma Black Belt notes out for this one :)

Microsoft Excel

From novice to tech pro — start learning today.

Experts Exchange Solution brought to you by

Enjoy your complimentary solution view.

Get every solution instantly with Premium.
Start your 7-day free trial.

Please state more clearly what you are trying to do, because I think you may be confused as to what a confidence interval for a sample mean is :)

Suppose you drew a random, unbiased sample of 50 items from a normally distributed population. Further suppose the following:

The confidence interval for your sample mean is a function of the sample mean, the sample standard deviation, and the confidence level.

Based on the above, first you would compute the standard error:

=sample_std_dev / sqrt(sample_size)

=15 / sqrt(50)

~ 2.12

Next, find the t value associated with 95% confidence and 50 degrees of freedom. This is about 2.01.

Multiply the two to get your margin of error, which in this example is ~ 4.26.

Now, your confidence interval is (sample mean) +/- (margin of error), or 95.74 - 104.26.

What this means is that we expect that there is a 95% chance that the true population mean is a value within the range 95.74 - 104.26.

Taking a bigger sample is not going to increase your confidence level. Rather, having a larger sample,

ceteris paribus, makes the width of the confidence interval narrower.Please see the attached file for an example of how I would do this.

Q-28009322.xlsx

Patrick