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Let

$$ S \sim N(\mu, \sigma^2) $$

be a normally distributed random variable with known $\mu$ and $\sigma^2$. Suppose, we observe

$$ X = \begin{cases} T & \text{if $S \ge 0$}, \\ -T & \text{if $S<0$},\end{cases} $$ where $T \in \mathbb{R}$. The probability distribution of $X$ is given by: $$ p(x) = Q\left(\frac{-\mu}{\sigma}\right)\delta(x-T)+Q\left(\frac{\mu}{\sigma}\right)\delta(x+T) $$

I want to optimize the value of $T$ such that $X$ conveys as much information about $S$ as possible.

My Attempt:

a. I tried minimizing the Kullback–Leibler divergence between the distribution of $X$ and $S$, but as mentioned here, it is not possible.

b. I tried to calculate the mutual information between the two distributions, it turned out to be independent of $\alpha$.

Is there any other way of formulating this problem? I feel quite confident that there must be such $T$ for which $X$ explains $S$ better, e.g., assume $\mu=10000$ then a value of $T$ near $10000$ will better explain $S$ than say $T=2$? One method in my mind was to match the moments of the two distributions but I am not sure if it is the optimal way in the sense of maximizing the information?

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You wrote "if $s\ge 0$". Could you have meant "if $S \ge 0$"? –  Michael Hardy Jul 1 '12 at 4:07
    
@MichaelHardy I thought we could define the function with small letters as well as capital letters because small letters show the specific values of random variable. –  ubaabd Jul 1 '12 at 5:26
    
One can write things like $\Pr(S=s)$ or $\Pr(S\ge s)$, but presumably your piecewise definition should be of another random variable. –  Michael Hardy Jul 1 '12 at 20:34
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1 Answer 1

up vote 1 down vote accepted

$P(S|X)$ is the same for any value of $T$. Hence, $X$ conveys the same information about $S$, no matter the value of $T$. No matter what $T$ you chose, you are only informing with $X$ if $S \geq 0$ or not.

A way for $X$ to be more informative about $S$ is to define it as:

$$ X = \left\{ \begin{array}{ll} 1 & \mbox{if $S \geq \mu$};\\ -1 & \mbox{if $S < \mu$}.\end{array} \right. $$

From your questions, you might be interested in Rate-distortion theory.

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can you please elaborate on what basis you mentioned $X$ is more informative about $S$ if we choose $T=1$. –  ubaabd Jun 30 '12 at 7:41
    
@ubaabd, any value of $T$ is equally informative. I'm saying that $X$ is more informative because it's value depends on $S \geq \mu$ and not $S \geq 0$. –  madprob Jun 30 '12 at 7:46
    
Oh i missed the change in 'if' conditions. Thanks –  ubaabd Jun 30 '12 at 7:48
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