If X- is zero, then there is no variation in X. Thus, there can be no covariation... The issue is logical, not computational. I can't think of any method that can show association in the absence of variation in one of the variables...
WBW On 21 Aug 2003, Garbriel wrote: > I have a question. Thanks a lot for your help, > It is about statistics in computing phi correlation coefficient: ( which > is a special formula for the Pearson product-moment correlation in the > particular case of two binary variable.) > > We used the method described in > > http://www.cmh.edu/stats/definitions/phi.htm > > See the following contigency table , > > Y+ Y- > ------------- > X+ |a11 | a12| > ------------ > X- |a21 | a22| > ----------- > > Defination of Phi, > > a11*a22 - a21*a12 > phi= --------------------------------------------- > sqrt((a11+a12)*(a21+a22)*(a11+a21)*(a12+a22)) > > phi>0.7 strong correlation. > phi>0.3 weak correlation > > > However, there is limitations in this calculation, > > since if X- is zero, so, a21=0, a22=0; > ************************************* > There will appear 0/0. > (and also big problem in p_value calculation). > > How to solve this? Is there any other method to quatitfy this when > denominator (4 sums) appear 0? ( I think phi correlation coeeficient is > not qualified to this situation.) And also compute p value or other > statistics term in this situation? > . > . > ================================================================= > Instructions for joining and leaving this list, remarks about the > problem of INAPPROPRIATE MESSAGES, and archives are available at: > . http://jse.stat.ncsu.edu/ . > ================================================================= > . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
