Would you be so kind to help me further. I already looked on the internet, textbooks… But I can’t find the answer to my question.

My working population is non normally distributed and from time to time I need to take a sample of it. Sometimes my sample exists of 100 units, at other times of 150 units.

Of my sample I calculate the mean and with bootstrapping I determine the sMean.

Since sMean = spop /√n I calculate the spop. So far so good, by using control charts (mean, spop) I can now see if there is anything happening to worry about.

By use of deviation calculation rules you can prove quite easily that the formula sMean = spop /√n also can be used for non normal distributions.

I play with the idea to make control charts using the trimmed mean and the 70%-value. Now I am not so sure anymore… (Frankly, I have my doubts wether you can do this for normal distributions as well) Can I use the same simple trick to remove the n out of my equation?

Let me put it like this: Is what I am writing valid? S70% = cte * spop /√n with a cte independent of the value of n? Strimmed mean = cte * spop /√n with a cte independent of the value of n?

To use my control charts I don’t need to know the value of my cte. For my s-chart I can just plot out cte* spop in function of my sample number. But I really need to get rid of the n!!!

I hope you understand what I am getting at. I would already be happy with a yes or a no, you can not do that anymore. If you can direct me to a place where I can find more information about this problem I would be even more gratefull.