Re: correlation of binary variables

[dennis roberts]

what if you had 8 people ... who took a two item test ... and, X = item 1 
score and Y = item 2 score ...
you have 8 paired observations ... yes ... they all count ... why would you 
think that some should be deleted?

here are the data

        -
     1.05+
         -                                                   5
 Y       -
         -
         -
     0.70+
         -
         -
         -
         -
     0.35+
         -
         -
         -
         -
     0.00+ 2                                                 *
           +---------+---------+---------+---------+---------+------x
        0.00      0.20      0.40      0.60      0.80      1.00
MTB > corr c1 c2

Correlations: x, Y

Pearson correlation of x and Y = 0.745

now, how you interpret this might be a problem ... since correlation is a 
measure of linear relationship between X and Y and ... i am not sure you 
can describe the above scatterplot as linear ...

but, the r = .745 in any case and yes ... you use ALL pairs of data

At 04:48 PM 4/2/2003, Galen Wilkerson wrote:
Hi,

I'm trying to get a definitive answer about this:

Suppose you have two binary random variables, and you get some data
from them:


When you are calculating the correlation coefficient of these two, do
you count the multiple occurances of the (1,1) and (0,0) pairs, or
just use them once?
Please respond by email.

thanks,
-Galen
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dennis roberts, educational psychology, penn state university
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