# Relation between cross entropy and joint entropy

Definition of cross entropy (Wiki link for details): $$H(p,q) = H(p) + \mathcal{D}_{KL}(p||q)$$

Definition of joint entropy: \begin{align*} H(X,Y) &= -\sum_x \sum_y p(x,y) \log p(x,y)\\ &= H(X) + H(Y|X) \end{align*}

What are the differences between these two? I'm having difficult to distinguish the concept of entropy for random variables versus the concept of entropy for distributions?

Are there any connections between these two? E.g., can we use joint entropy to prove cross entropy formula, if not, how would you derive cross-entropy formula?

• Firstly, note that cross entropy is defined only for distributions. One thing to note is that $H(p,q) = -\sum_x p_x \log q_x$ (to show this, expand both $H$ and $D$ and use $\log a/b = \log a - \log b$). Thus, cross entropy is the best possible expected length of the encoding of a source when the codebook is designed to be optimal for $q$. $H(X,Y)$ is operationally different - it is the minimum possible expected length of an encoding of $(X,Y)$ together. In any case, the two quantities are not implicitly related. – stochasticboy321 Nov 5 '17 at 21:08

Unfortunately, practitioners use nearly identical notation for both cross entropy and joint entropy. This adds to the confusion. I will distinguish the two by using
$$H_q(p) - \text{for cross entropy}$$ and $$H(x,y) - \text{for joint entropy}$$

Cross Entropy

Cross Entropy tells us the average length of a message from one distribution using the optimal coding length of another. For example,

$$H_q(p) = \sum_{x} p(x) \log\bigg(\frac{1}{q(x)}\bigg)$$

Here, $\log\big(\frac{1}{q(x)}\big)$ is the optimal coding length for messages coming from the $q$ distribution.

While $p(x)$ is the cost of sending message $x$ from $p$.

Putting these together we can interpret $H_q(p)$ as the average cost of sending messages from $p$ using the optimal coding length for $q$.

Joint Entropy

Joint Entropy tells us the average cost of sending multiple messages simultaneously. Or perhaps more intuitively, the average cost of sending a single message that has multiple parts. For example,

$$H(x,y) = \sum_{x,y} p(x,y) \log \bigg(\frac{1}{p(x,y)} \bigg)$$

Here, $\log \big(\frac{1}{p(x,y)} \big)$ is the optimal coding length for messages coming from the $p$ distribution.

While $p(x,y)$ is the cost of sending the message $x,y$ from $p$.

It may be clear from this that joint entropy is merely the extension of entropy to multiple variables. In addition, multiple random variables are often represented as vectors $\bf x$. In which case, calculating the entropy of its distribution $p({\bf x})$ results in whats 'appears' to be a 'regular' entropy.

$$H(p)=\sum_{{\bf x}}p({\bf x})\log \bigg(\frac{1}{p({\bf x})} \bigg)$$

I make this last point to demonstrate that distinguishing between entropy and joint entropy may not be very useful.

Further Resources

Chris Olah wrote an excellent article on Information Theory with the goal of making things visually interpretable. It is called Visual Information Theory. The notation I adopted came from him. The distinction and relation between cross entropy and joint entropy is demonstrated via figures and analogies. The visualizations are very well done, such as the following which demonstrates why cross entropy is not symmetric. Or this one which depicts the relationship between joint entropy, entropy, and conditional entropy. • I agree that the notations used in information theory are confusing. But the notation $H_q(p)$ does not tell us the random variable that we are considering. I feel that the idea of cross entropy is a little awkward because by considering two "distributions", we no longer live in a single probabilistic model. We are actually considering two probabilistic models with the same sample space but different probability rules. – W. Zhu Feb 20 '19 at 8:36
• W.Zhu, I agree that cross entropy can be a strange notion. However, for now, I believe we can determine the random variable from $H_q(p)$. In the definition, the expectation is performed over $p$. In that way, the notation $H_q(p)$ explicitly implies that the random variable comes from the distribution within the parenthesis $(\cdot)$. – Nathan Crock Feb 28 '19 at 17:34
• I would argue in favor of writing $H_p(q)$ since it would be more consistent with the standard notation employed for the expected value: $H_p(q):=\mathbb E_p [- \log {q}]$. – Immanuel Weihnachten Sep 24 '19 at 11:09