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DotFourier
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Homework Statement
Let $$S_{1}^2, S_{2}^2, S_{3}^2$$ be sample variances of sample size $$n_{1}, n_{2}. n_{3}$$ respectively. The populations have means $$\mu_{1}, \mu_{2}, \mu_{3}$$ respectively with common variance $$\sigma^2.$$
When is $$\pi_{1}S_{1}^2 + \pi_{2}S_{2}^2 + \pi_{3}S_{3}^2$$
unbiased?
Homework Equations
The Attempt at a Solution
I was under the impression that the sample variance was always unbiased. Wouldn't the expected value of the above expression always yield the same expression?[/B]