# How to understand units of information [duplicate]

In information theory, specifically in information content, I am struggling to conceptualise what the unit of measurement actually is. I have read quite a few similar questions and worked through the derivation in Probability Theory: The Logic of Science, E.T Jaynes but still struggling to piece together the information.

The part I am struggling with is why the information content takes the form.

$$I_X(x) = log\biggl(\frac{1}{p_X(x)}\biggr)$$

From what I have read so far, the use of the logarithm function makes sense, but $$\bigl(\frac{1}{p_X(x)}\bigr)$$ is where I am stuck. In plain english, I understand this to mean the quantity of information transmitted, or level of surprise when some event occurs. But I don't follow why this would takes this form. One idea I have trying to confirm (unsuccessfully so far) is that if that if some event $$E$$ occurs with probability $$P$$, then the amount of information received when this event occurs could be considered as $$1 - P$$.

If this is true, while also satisfying the requirements outlined by Shannon, is information content defined as above due to the following?

$$I_X(x) = log\bigl(1\bigr) - log\bigl(p_X(x)\bigr)$$ $$I_X(x) = 0 - log\bigl(p_X(x)\bigr)$$ or $$I_X(x) = log\biggl(\frac{1}{p_X(x)}\biggr)$$

I'm not sure if this is just assumed as being obvious, or a coincidence, but I haven't yet found an explanation that formally describes information content in this way (i.e. why information is $$\frac{1}{p_X(x)}$$).

• Maybe my answer here helps. Aug 29, 2019 at 14:43
• @kjetilbhalvorsen, thanks for your help. In your answer, where you say "we throw in a minus sign to get a positive number", is it just a coincidence that $log\biggl(\frac{1}{p_X(x)}\biggr) = log(1) - log(p_X(x)) = -log(p_X(x))$? Or do we end up with a minus sign because we start by saying the amount of information from sampling something with probability $P$ may be measured as $1 - P$? Then following the guidance of Shannon, this equates to $log(1) - log(log(p_X(x))$ or just $-log(p_X(x))$? Aug 29, 2019 at 16:02

Now suppose you want to explain someone how unlikely or hard to predict a random event with probability $$P(X)$$ is. You should proceed in a similar fashion. Choose a known random event that, like the "nearby" object in the example of the football field, will serve as a comparison. The simplest choice is for this "unit of measure" is probably the toss of a coin. If the person is not familiar with coin-tossing, you can easily show it to him/her.
An event with probability $$P(X)$$ then corresponds to $$\log_{\frac{1}{2}} P(X)$$ tosses of coins. For example, the toss of two coins is twice as hard to predict as the toss of one coin. Similarly, any outcome when you roll a die ($$P(X) = 1/6$$) is approx 2.6 times harder to predict than the coin toss, etc.
From the properties of logarithm you have $$\log_{\frac{1}{2}} P(X) = - \log_2 P(X) = \log_2 \frac{1}{P(X)}$$