How to Determine Sample Size

Manu – You want to know the sample size necessary to show that there is an acceptable amount of variation – if you have no variation you just have an incapable measurement system.
You need to specify what amount of variation you need to be able to detect, and the power you want to assign the test.  Plug the numbers into Minitab or other program, and you will get your sample size recommendation.

Hi Manu,
Since your measurement is a continuous variable, I’m wondering if the following would work:
1. Determine the standard deviation (SD) of the sample measurements you’ve collected thus far.
2. Determine the degree of desired precision (P) for your assessment purposes (e.g., to the nearest 1 Newton? 5 Newtons? 10?)
3. Plug this information into the following formula:

Required sample size = (2*SD/P)^2 (that’s 2 times the standard deviation, divided by the desired precision, quantity squared).
We used this method in a recent Six Sigma project when we were collecting baseline data on patient readiness times.
Hope this helps!
Patrick

Mighty Mike –
Did you just state “prove your hypothesis”?  Shame!