st: Kendalls tau-b difference between ktau and tabulate taub

["Kornak, John" <john.kornak@ucsf.edu>]

Hi Statalist members,

I am trying to figure out why I get different results using the stata function ktau versus tabulate with the taub option.

For example if I have the following dataset

. list

     +-------+
     | x   y |
     |-------|
  1. | 3   5 |
  2. | 6   7 |
  3. | 2   3 |
  4. | 9   4 |
  5. | 7   5 |
  6. | 4   2 |
     +-------+

. 

then using ktau I get

. ktau x y, stats(taub se)                      [I have snipped the output to the relevant parts below]

Kendall's tau-b =       0.1380
    SE of score =       5.228   (corrected for ties)
. 

Within tabulate; (tabi 3 6 2 9 7 4\5 7 3 4 5 2, taub) I get

 Kendall's tau-b =  -0.1667  ASE = 0.115

If I transpose rows and columns I still get the same (i.e. with tabi 3 5\6 7\2 3\9 4\7 5\4 2, taub)

Even cases like correlating 1,2,…,n with itself (tabi 1 1\2 2\3 3\4 4\ 5 5, taub) in tabulate gives an unexpected answer to me of 0. 

I am using Stata 13 on a macbook pro running OS X v. 10.8.5.

Any insights into why I get differing answers would be very much appreciated. I expect I am misinterpreting what taub is supposed to be doing in tabulate.

Thanks for any help

John


---------------------------------------------------- 
John Kornak, PhD
Associate Professor in Residence
Department of Epidemiology and Biostatistics
University of California, San Francisco
UCSF Box 0560
185 Berry Street, Lobby 5, Suite 5700
San Francisco, CA 94107-1762
Tel: 415-514-8028
Fax: 415-514-8150
Email: john.kornak@ucsf.edu

 



*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/faqs/resources/statalist-faq/
*   http://www.ats.ucla.edu/stat/stata/