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I have studied about the basic probability theory. Now I am studing about entropy from information theory point of view from Bose's Information Theory, Coding and Cryptography.
I have not understood the difference between mutual and conditional information.

Suppose there are two random variabes $X\;,Y$ with outcomes $x_i$ where $i=1,2,...,n$ and $y_j$ where $j=1,2,...m$.
The Mutual Information $I(x_i;y_j)$ between $x_i$ and $y_j$ is defined as $I(x_i;y_j)=\log \frac{P(x_i|y_j)}{P(x_i)}$.

The conditional information between $x_i$ and $y_j$ is defined as $I(x_i|y_j)=\log \frac{1}{P(x_i|y_j)}$

They give an example for mutual information in the book. Screenshot of Example 1.3

We can see that if $p=0$ (meaning the channel is ideal), then knowing the input is $0$ or $1$ gives us all the information about the output (which is also $0$ or $1$ respectively).

For $p=0$, $I(Y=0;X=0)=\log_2 2(1-p)=1$.

Also, $I(Y=0|X=0)=-\log_2(P(Y=0|X=0))=-\log_2(1-p)=-\log_2(1)=0$

Q-1 I am not able to understand intuitively what the difference is between $I(Y=0;X=0)=1$ and $I(Y=0|X=0)=0$. What is the difference between mutual and conditional information?

Q-2 For $p<0.5$, $I(Y=0;X=0)=\log_22(1-p)<0$. What does the negative information mean?

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  • $\begingroup$ Note that most sources would call $\operatorname{I}(x_0; y_0)$ the pointwise mutual information. $\endgroup$ – Arya McCarthy Mar 31 at 19:18
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I think using $\operatorname{I}$ for both conditional information and pointwise mutual information might confuse things, so I'll use $h$ for conditional information and $\operatorname{pmi}$ for PMI.

Q1: pointwise mutual information versus conditional entropy

The 'conditional information' $h(y_0 \mid x_0)$ is a measure of uncertainty: how surprised are we that $Y = 0$ given that $X = 0$? In a noiseless channel, we're not surprised at all, which is why the value is $0$. If we already know that $X = 0$ in a noiseless channel, then finding out that $Y=0$ gives no additional information. As the noise increases, so will our surprise if $Y=0$ given that $X=0$.

By contrast, $\operatorname{pmi}(x_0; y_0)$ is a measure of the shared information content. Given what we already know about the individual events $x_0$ and $y_0$, how much does one tell us about the other?

The two quantities can be related by this equality: $\operatorname{pmi}(x_0; y_0) = h(y_0) - h(y_0 \mid x_0)$. Knowing that the event $x_0$ occurs may make $y_0$ more or less surprising. The $\operatorname{pmi}$ function quantifies this change in surprise in bits.

Q2: meaning of negative PMI

When the pointwise mutual information is negative, this means that $x$ and $y$ co-occur less frequently than would be expected if they were independent. This can be made clearer by rewriting the formula:

$$ \operatorname{pmi}(x_0; y_0) = \log \frac{p(y_0 \mid x_0)}{p(y_0)} = \log \frac{p(y_0 \mid x_0) p(x_0)}{p(y_0)p(x_0)} = \log \frac{p(y_0, x_0)}{p(y_0)p(x_0)} $$

The fraction is less than 1 when the two events don't tend to co-occur as often as if they were independent; the log of the fraction is thus negative.

Because of the very-high noise in the channel, we don't expect $x$ and $y$ to have the same value; it's more likely expect the channel to corrupt $x$.

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  • $\begingroup$ Thanks for the reply, the conditional information $h(y_o,x_o)$ is the surprise we feel (or the information) if we come to know the information $x_o$ first. If the channel is ideal or toward ideal, then if $x=0$, there is high probability that $y=0$, so we get less suprise after knowing $x=0$. But still intuitively I am not able to understand about pointwise mutual information. It is a measure of shared information content. What is meant by that? Can you please explain it in somewhat detail. $\endgroup$ – Iti Apr 1 at 6:14
  • $\begingroup$ Knowing $X$ reduces your uncertainty about $Y$ by $\operatorname{pmi}$ bits. The same is true about the uncertainty in $X$ from knowing $Y$. That’s what the PMI is measuring. $\endgroup$ – Arya McCarthy Apr 3 at 15:22

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