## Information Theory

author: David MacKay, University of Cambridge
published: Nov. 2, 2009,   recorded: August 2009,   views: 14781
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1 raghvendra, December 1, 2010 at 12:42 a.m.:

realy very good

2 eliot, January 7, 2012 at 8:43 a.m.:

skillful lecturer

3 Bart Alder, August 8, 2012 at 7:28 a.m.:

Excellent.

4 mahdie, February 20, 2013 at 7:34 p.m.:

very good

5 nuur, August 16, 2013 at 3:53 p.m.:

test

6 Julien, April 16, 2018 at 7:59 p.m.:

I have issue with the example using cards. The correct answer is 1/2. Proof:

A card is a pair of colors:

c1 = (R, R)
c2 = (R, W)
c3 = (W, W)

We choose a card (x,y) at random.

I see red, so I know x = R. He then asks P(y = R | x = R)

P(y = R | x = R) = P(x = R, y = R) / P(x = R)
= p(card = c1) / p(card = c1 or card = c2)
= p(card = c1) / (p(card = c1) + p(card = c2))
= 1 / 2

Why does he say 2/3?