Data Compression : Finding Redundancy
Most types of computer files are fairly redundant -- they have the same information
listed over and over again.
file-compression programs list information once and then refer back to it
whenever it appears in the original program.
In John F. Kennedy's 1961 inaugural address :
"Ask not what your country can do for you -- ask what you can do for
your country."
17 words, made up of 61 letters, 16 spaces, one dash and one period : total
file size of 79 units.
"ask", "what", "your", "country",
"can", "do", "for", "you" appear twice
Source: http://computer.howstuffworks.com/file-compression1.htm