The simplest statistical model for the source is to assume that each letter that is generated by the source is independent of every other letter, and each occurs with the same probability. We could call this the ignorance model, as it would generally be useful only when we know nothing about the source. The next step up in complexity is to keep the independence assumption, but remove the equal probability assumption and assign a probability of occurrence to each letter in the alphabet. For a source that generates letters from an alphabet
