Statistical Analysis of a Mass Distribution

Hi all,

I'm looking at the analysis of a sample of a mixture of materials, with the aim of determining the minimum sample size required in order to conform to a certain error. The basics are:

- I assume that the population of my mixture is infinite/very large

- There are N fractions, with the ith fraction having an estimated mass fraction and average mass of $\displaystyle x_i$ and $\displaystyle m_i$ respectively. Obviously, the sum of all the mass fractions is 1.

What could I use to determine my minimum sample size with this info? Or would I need something else? I have tried to use some sort of Binomial approximation to the number fractions, but I really don't thing it's particularly good! Is there any way I could do it by considering the sample as a combination of a series of normal distributions?

Thanks in advance - I've been banging my head on the keyboard for about three weeks over this!