# Entropy and Quantity of Information

From: David Cressey <cressey73_at_verizon.net>
Date: Fri, 11 Jan 2008 18:41:40 GMT
Message-ID: <ELOhj.365\$rG.108_at_trndny02>

Instead of hijacking another topic, I'll start this topic.

David BL suggested that a way to quantify information is the number of bits needed to encode, allowing for compression. I said I preferred entropy as the measure of information, and suggested that the two measures might in some way be equivalent. Someone else recalled the concept of entropy from the study of statistical mechanics and thermodynamics in physics.

The use of the word "entropy" definitiely comes from physics. The formulas that Shannon (qv) determined for the amount of information in a message bear an uncanny resemblance to the formulas for entropy in physics. I personally don't believe that this is mere coincidence.

Next, I talked a little about entropy being context sensitive. There is a much more precise way than mine of stating this, but I've forgotten what it is. I think it goes something like this: the self information in a message is context insensitive but the mutual information between a pair of messages is dependent on both of them. My use of the word "database" in place of message in my earlier comment is a little sloppy in this regard.

Next, the use of logarithms, and the number of bits needed to store (represent?) a message are closely related. If we have 8 equally likely messages, it will take precisely 3 bits to store one message. Note that log base 2 of 8 is 3. If you use the formulas for entropy, and use 2 as the base for the logarithms, the unit of information comes out to be precisely the bit.

There is a form of compression called Huffman encoding on an ensemble of messages that takes advantage of mathematical expectation to compress, on the average, an ensemble of messages when the eight possible messages are not a priori equally likely. If for example, message number 1 is 50% likely, you and use a message consisting of 1 bit to represent it, and use somewhat longer messages for the other 7 possible messages. I don't want to belabor the point in cdt, but there are ample references for Huffamn encoding on the web. If you study Huffman coding in detail, you'll start to get logarithms into the picture.

All of this goes back to the 1960s, and some of it to the 1940s. Is entropy still widely used in information science? Is it relevant to database theory? Received on Fri Jan 11 2008 - 19:41:40 CET

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