where "pi" is the probability that an information source will produce the "i" th symbol from an "r"-symbol alphabet and is independent of previous symbols. (thus "i" runs from 1 to "r" ). "H" is then a measure of the expected amount of information (log "pi" ) missing before the symbol is known or measured, or, alternatively, the expected amount of information supplied when the symbol becomes known. The set of messages of length "N" symbols from the source will then have an entropy of N*H.