# Statistically Determining how many Tests to conduct.

J

#### JLRothbauer

Question: How to determine how many samples needed to ensure that you have statistical similarity, i.e. one more sample is just more numbers..

Background info....
I am conducting a series of tests on our newly complete Co-Extrusion Film line. We have produced a tremendous amount of samples and we are currently working our way through them.
Our mission is to measure the thickness of each individual layer in the 3 layer film using a microscope.
We have several hundred samples 82 inches wide that are tested every inch.
For each individual test there are three measurement made: Thickness Layer 1, Thickness Layer 1 + 2, Total Thickness.
So, in the end, you have a web profile indicating the film's thickness.
How do you determine how many samples are necessary. Each sample takes over 2 hours to do, so the difference between 5 and 10 is a huge amount of time.
When is, for example, 5 samples worth of data no better than 6 samples worth of data on a statistical level. How do I determine this?
Ugh, what an ugly post. I must apologize that I can't find the proper way to put my problem into words anyone can understand.

Well I thank you all in advance; any help would be greatly appreciated.

##### One of THE Original Covers!
Do you have access to "Quality Planning and Analysis", Juran/Gryna?

You are testing a hypothesis and Chapter 10 in this book details what you need to know far better than I can put it to words here.

Regards,

Kevin

J

#### JLRothbauer

Thanks.

No, I don't actually have access to that book, but I do have others I can look into.

I have some statistical training under the belt, but my problem has always been application of the techniques when/where. I see that the isse is rearing its ugly head yet again.

I will go through my book on the topic of Testing an Hypothesis and relearn the technique.

Thanks for the advice.