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I have two multivariate normal distributions (say X and Y) fitted on existing data. With new data I want to compute the probability that sample is from distribution X instead of Y. It can only be one of these two distributions.

Any ideas on how I can go about doing this?

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    $\begingroup$ To compute such a probability you would need a prior probability on the two. From there it's a simple application of Bayes theorem $\endgroup$ – Glen_b -Reinstate Monica Sep 10 '16 at 1:21
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If the density functions of $X$ and $Y$ are $f_X$ and $f_Y$, and the prior probabilities of the data vector $d$ being drawn from distribution $X$ and $Y$ are $p_X$ and $p_Y$ respectively then the probability the data came from distribution $X$ can be found using Bayes theorem:

$$ \frac{f_X(d) p_X}{f_X(d) p_X + f_Y(d) p_Y} . $$

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