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.