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    Normal sufficient statistics



    I'm trying to work out the sufficient statistic for a normal distribution. I've figured out the statistic for when mu is unknown as that's simply x-bar. But when I thought that just S^2 was the sufficient statistic for sigma^2, I was told that was incorrect. Additionally, when both are unknown it's not just the two sufficient statistics of the constituents like it is with a Gamma distribution.

    I know I'm missing something here but I'm not sure what...

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    Re: Normal sufficient statistics


    Quote Originally Posted by jareddm View Post
    I'm trying to work out the sufficient statistic for a normal distribution. I've figured out the statistic for when mu is unknown as that's simply x-bar. But when I thought that just S^2 was the sufficient statistic for sigma^2, I was told that was incorrect. Additionally, when both are unknown it's not just the two sufficient statistics of the constituents like it is with a Gamma distribution.

    I know I'm missing something here but I'm not sure what...
    If you extend the Fisher-Neyman factorization theorem to two parameters you can obtain:

    Y_{1}=\sum X_{i}^{2} and

    Y_{2}=\sum X_{i}

    as the joint sufficient statistics. And, thus, the single-valued functions are also joint sufficient statistics, namely,

    \bar{X}=\frac{Y_{2}}{n} and

    S^{2}=\frac{Y_{1}-\frac{Y_{2}^{2}}{n}}{n-1}.

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