In information theory, entropy is a measure of the uncertainty in a random variable. In this context, the term usually refers to the Shannon entropy, which quantifies the expected value of the information contained in a message.
The formula for entropy was introduced by Claude E. Shannon in his 1948 paper "A Mathematical Theory of Communication".
Minus is used because for values less than 1 logarithm is negative, however, since
formula can be expressed as
is also called an uncertainty or surprisal, the lower the probability , i.e. → 0, the higher the uncertainty or the surprise, i.e. → ∞, for the outcome .
Formula, in this case, expresses the mathematical expectation of uncertainty and that is why information entropy and information uncertainty can be used interchangeably.
This calculator computes Shannon entropy for given probabilities of events
This calculator computes Shannon entropy for symbol frequencies of given message.