The Measurement of Information
the amount of information carried inside a message is determined by its probability, not often 50% to 50%, in the set of all possible messages. When probability approaches zero, information approaches infinity; when probability approaches 1, information approaches zero (since you have no uncertainty).
If a question has N possible answers with probability of Pi ( i = 1,2,……N), the information in a message will be measured:
Pi: probability of state i