gjm comments on The usefulness of correlations - Less Wrong

13 Post author: RichardKennaway 04 August 2009 07:00PM

You are viewing a comment permalink. View the original post to see all comments and the full post content.

Comments (52)

You are viewing a single comment's thread.

Comment author: gjm 04 August 2009 10:13:36PM 6 points [-]

Simplicio collects some experimental data consisting of a great many pairs (X,Y) and with high confidence finds a correlation of 0.6 between X and Y. So given the value y of Y, his best prediction for the value of X is 0.6y.

Eh?

That's just not what correlation means. (If we have, say, X=0.6Y or X=100Y or X=0.0001Y, exactly in each case, then the correlation is 1. The correlation tells you nothing about the coefficient in the relationship.)

Comment author: Eliezer_Yudkowsky 04 August 2009 11:47:17PM 0 points [-]

Presumably X and Y have been converted to canonical form with mean 0, sd 1.

Comment author: RichardKennaway 04 August 2009 10:55:35PM 0 points [-]

Yes, that was an error. I was thinking of the case where X and Y are both normalised to have s.d. 1, in which case the regression line is indeed Y = cX, but that isn't the case here. In general, the line is Y = bcX/a where the standard deviations of X and Y are a and b.