Information, or self-information, in information theory is a basic element to measure the amount of information quantitatively. As an example, information can be measured by the unit of bit as follows

$ I(X) = \log_2 \left( \frac{1}{p(X)} \right) $

where $ X $ is the event, $ p(X) $ is the probability of event $ X $. If the event is an unexpected or less expected to happen, the amount of information is higher while the frequently happened events have low values of $ I(X) $.

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