# Talk:Conditional independence

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## Redundant Intro

The first paragraph of this article says the same thing, three times. I am not sure what is the best way to simplify it, but it is confusing enough already. —Preceding unsigned comment added by 72.29.211.19 (talk) 01:06, 11 May 2011 (UTC)

http://www.dcs.qmul.ac.uk/~norman/BBNs/Independence_and_conditional_independence.htm Davyzhu (talk) 06:05, 27 February 2008 (UTC)

Conditional Dependence links to the butterfly effect? That seems kind of absurd! That article hardly discusses conditional dependence and is totally different. I think we should have this page discuss both conditional dependence and independence. I changed the link to link here, and renamed the link to the butterfly effect.Epachamo 00:16, 20 April 2007 (UTC)

## Graphic of Squares

I cannot verify that P(AUB|not C) in the first figure is equal to P(A|C)*P(B|C). I count that P(AUB|not C) is 4/37, P(A|notC) is 12/37, and P(B|notC) is 12/37. That doesn't work out. Did I miss something? 76.8.64.166 (talk) 20:51, 16 January 2008 (UTC)

Agree, the graphic is incorrect. It would be valid if the grid were 6x8 instead of 7x7, with other areas unchanged, then fractions are 12/36 x 12/36 = 4/36 167.136.242.40 (talk) 22:57, 18 February 2008 (UTC)dfry

Yeah, the graphic is incorrect !!!

Make that 4 unconditionally independent votes for incorrect.

5!!!!! —Preceding unsigned comment added by 195.219.143.2 (talk) 08:27, 20 April 2009 (UTC)

—Preceding unsigned comment added by 58.161.42.51 (talk) 00:54, 8 May 2008 (UTC)

Actually the link to the jpg (figure named CondicionalmenteIndependientes.JPG‎) IS correct... I do not know why it is shown this wrong one... —Preceding unsigned comment added by 193.204.89.132 (talk) 07:48, 28 May 2009 (UTC)

I no not know if this is the correct place to say this, but I have modified the current SVG figure to make it again correct ( like in the original picture, where the first figure grid is 6x7 ). Unfortunally, I have tried to upload the figure as a replacement of the actual one but I have only been able to upload it as a new picture. As I do not want that the history of the actual file gets lost, I have not finished the uploading. I think that I can not modify it because I have not written enough in wikipedia and I do not have permissions. Could someone help me? Cesarth (talk) 08:02, 10 April 2015 (UTC)

## Independence

I have been told that two variables can be indendant, but not conditionally independant. For example, if a variable Z is dependant on two variables X and Y, and X and Y are independant, then X and Y are *not* (or may not be) independant given Z. Is this true? If it is it should be explained in this article. Fresheneesz 01:55, 1 December 2007 (UTC)

Example proving that two variables may not be conditionally independant even if they are unconditionally independant:
X – 0 or 1 (random coin flip)
Y – 0 or 1 (random coin flip)
Z – sum of X and Y (can be 0, 1, or 2)
X and Y are obviously independant. P(Z==0)=0.25, P(Z==1)=0.5, P(Z==2)=0.25 . Knowing what Z is will enable you to know something about what X is if you know what Y is - in some cases allowing you to know exactly what X is. Thus X and Y aren't conditionally independant given Z. Fresheneesz 03:30, 1 December 2007 (UTC)

## Unclear Baysian networks

The section on Baysian networks is exteremly unclear, and poorly constructed. It is always a bad idea to begin with an example. The addition of the "frequentist" further confuses the section. Fresheneesz 02:11, 1 December 2007 (UTC)

I disagree -- it is always a GOOD idea to begin with an example! People are pretty good at generalising from a few worked examples, but most (even those of us with PhDs) find it difficult to understand things expressed as generalities. When I was teaching at a university I always started with an example and then went on to the general theory, and the students definitely approved of this technique. Unfortunately most textbooks do the opposite. 88.110.89.8 (talk) 18:34, 11 January 2010 (UTC)

## Incorrect definition

"Two random variables X and Y are conditionally independent given a third random variable W if for any measurable set S of possible values of W, X and Y are conditionally independent given the event [W ∈ S]." — This is incorrect. Boris Tsirelson (talk) 14:07, 7 December 2008 (UTC)

I've fixed it. I'm not sure if we've got a good account of conditioning on a sigma-algebra. There's a section in the article titled conditional expectation that treats the topic, but it's fairly terse and does not mention the role of the Radon–Nikodym theorem. Michael Hardy (talk) 14:47, 7 December 2008 (UTC)
And please remove the obsolete "measurable set S of possible values of W". Yes, I know that such conditioning is not an easy matter; in fact, this is why I've created the Conditioning (probability) page. Boris Tsirelson (talk) 15:03, 7 December 2008 (UTC)
Now I did it myself. Boris Tsirelson (talk) 11:38, 16 December 2008 (UTC)

## Properties are rubbish

Practically all rules of conditional independence are rubbish and meaningless (unless a definition is added, what it means, when to the left or right of the relation symbol two sets show up and are seperated by a comma). —Preceding unsigned comment added by 93.207.223.32 (talk) 15:36, 7 May 2010 (UTC)

And now, after the edit by Ninjagecko of 21 October 2010, they are even more rubbish and meaningless than before. Boris Tsirelson (talk) 19:35, 21 October 2010 (UTC)