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st: RE: simple program to compare spearman coeffs using bootstrap - please help!


From   "Garth Rauscher" <garthr@uic.edu>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: simple program to compare spearman coeffs using bootstrap - please help!
Date   Thu, 29 Apr 2010 07:24:44 -0500

Andrew -try this


program drop spearmandiff
program define spearmandiff, rclass
capture drop def1 def2 diff 
spearman lastmam lastcbe
return scalar def1=r(rho)
spearman lastmam lastroutine
return scalar def2=r(rho)

return scalar diff =return(def1)-return(def2)

end

use dataset_name, clear
bootstrap def1=r(def1) def2=r(def2) diff=r(diff), reps(100) : spearmandiff
estat bootstrap , bc


Garth Rauscher
Associate Professor of Epidemiology
Division of Epid/Bios (M/C 923)
UIC School of Public Health
1603 West Taylor Street
Chicago, IL 60612
ph: (312)413-4317
fx:  (312)996-0064
em: garthr@uic.edu
 
Date: Thu, 29 Apr 2010 02:12:02 -0400
From: "Chong, Qi Lin Andrew" <qchong@middlebury.edu>
Subject: st: RE: simple program to compare spearman coeffs using bootstrap -
please help!

Hi all, 

sorry to bother again. This is a rudimentary program for what I'm trying to
do. I have put the data in the form eff1a eff1b eff2a eff2b where these
correspond to the same observation (in this case they are four efficiency
scores attached to a firm). I just want to run a spearman coeff on eff1a
eff1b and eff2a eff2b separately, and then take the difference. 

Can anyone help with the basic problems in this program? 

capture program drop spearmandiff
program define spearmandiff, rclass
spearman eff1a eff1b
return scalar def1=r(rho)
spearman eff2a eff2b
return scalar def2=r(rho)
gen diff = def1-def2
save diff 
end

bootstrap diff=r(diff), reps(1000) : spearmandiff

The program is wrong but it illustrates that I just want to draw a sample,
do a spearman test on the first 2 and the last 2 data pairs, and then
compare the differences. The final result for bootstrap should give me a
mean difference and a SD. 

I basically just want to calculate the statistical signficiance of the
difference between the two spearman coefficients. 1) I am not sure how to
save the different values of diff that I get from drawing the different
bootstrap samples, and 2) how to make sure they end up in the final output
for the multiple bootstrap sampling. Will this give me the confidence
interval I require to evaluate its statistical significance? 

Thanks for any help you can render! 

Andrew 

________________________________________
From: owner-statalist@hsphsun2.harvard.edu
[owner-statalist@hsphsun2.harvard.edu] On Behalf Of Roger Newson
[r.newson@imperial.ac.uk]
Sent: Wednesday, April 28, 2010 8:28 AM
To: statalist@hsphsun2.harvard.edu
Subject: Re: st: comparing differences in Kendall's tau or Spearman's
coefficient using somersd and/or bootstrap

Yes, this sounds like what you ought to be doing. Correlations between
X1 and Y1 can only be compared with correlations between X2 and Y2 using
firms with all 4 variables.

I hope this helps.

Best wishes

Roger


Roger B Newson BSc MSc DPhil
Lecturer in Medical Statistics
Respiratory Epidemiology and Public Health Group
National Heart and Lung Institute
Imperial College London
Royal Brompton Campus
Room 33, Emmanuel Kaye Building
1B Manresa Road
London SW3 6LR
UNITED KINGDOM
Tel: +44 (0)20 7352 8121 ext 3381
Fax: +44 (0)20 7351 8322
Email: r.newson@imperial.ac.uk
Web page: http://www.imperial.ac.uk/nhli/r.newson/
Departmental Web page:
http://www1.imperial.ac.uk/medicine/about/divisions/nhli/respiration/popgene
tics/reph/

Opinions expressed are those of the author, not of the institution.

On 28/04/2010 05:37, Chong, Qi Lin Andrew wrote:
> Hi all,
>
> thanks a lot to Dr. Newson for responding to my question. To him and all
others, I am unsure how I should proceed with the bootstrap command. As Dr.
Newson explained accurately, I am "trying to measure the tau-a correlation
between "Efficiency definition 1" (ED1) in Year A and ED1 in Year B, and
then measure the tau-a correlation between "Efficiency Definition 2" (ED2)
in Year A and ED2 in Year B, and then calculate a confidence interval for
the difference between the 2 taus."
>
> If I were to reorder the data set so that each firm has "ED1 YearA" "ED1
YearB" "ED2 YearA" "ED2 YearB" lined up next to each other, etc, ed1a ed1b
ed2a ed2b, how should I go about estimating this interval?
>
> If it is easy to give me the exact commands, I would very much appreciate
it, but if not any advice would be much appreciated. I am trying to
calculate the statistical significance of their difference, so should I be
drawing bootstrap samples of firms with ed1a ed1b ed2a ed2b all together?
And then calculating one difference between ed1a ed1b&  ed2a ed2b for each
sample?
>
> Would this involve some kind of programming using bootstrap alone and w/o
somersd? Also, does it matter that Spearman is in terms of RANKS, so if I
happen to draw the same sample twice from say the 6000 observations I have,
then there would be a tie in the bootstrap drawn sample (6000 obs from
original 6000 but with replacement)? Would this tie be a problem?
>
> Many thanks for your help!
>
> Andrew
>
>
>


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