Bookmark and Share

Notice: On March 31, it was announced that Statalist is moving from an email list to a forum. The old list will shut down at the end of May, and its replacement, statalist.org is already up and running.


[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: st: RES: generating a variable with pre-specified correlations with other two (given) variables


From   Tirthankar Chakravarty <tirthankar.chakravarty@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: RES: generating a variable with pre-specified correlations with other two (given) variables
Date   Wed, 31 Aug 2011 05:00:29 -0700

This question has appeared a few times before - in that you want to
create a variable with a pattern of correlation with _existing_
variables, which -corr2data- does not do. In an example where means
are normalised to zero, this can be had by solving a system of linear
equations in appropriate expectations.

Suppose you generate a variable as

Z = a*X+ b*Y ---(0)

where a, and b are constants to be determined. Then you can derive the
following identities under the zero mean assumption:

Cov(Z, X) = a*Var(X) + b*Cov(X, Y)  ---(1)
Cov(Z, Y) = b*Var(Y) + a*Cov(X, Y)  ---(2)

Here you know everything (you set Cov(Z, X) and Cov(Z, Y)), and this
is a system of two equations in two unknowns, a and b. Solve them and
generate your variables as in equation (0).

So for example, if I have Cov(X, Y) = .6, and Var(X)=Var(Y)=1, then a
=0.15625 , b=0.40625.
/************************************/
mat mCov = (1, .6\ .6, 1)
// generate x and y
corr2data x y, cstorage(full) cov(mCov) n(100000) clear
// generate z based on current sample of x and y
g z = .15625*x+.40625*y
corr, covariance
/************************************/

All these calculations are assuming zero means - more tedious algebra
will allow you to generalise.

T

On Wed, Aug 31, 2011 at 3:53 AM, Henrique Neder <hdneder@ufu.br> wrote:
> Try corr2data:
>
> matrix C = (1,0,.80,-.80\0,1,0,0\.80,0,1,-.80\-.80,0,-.80,1)
> corr2data hsperc corzer1 corpos1 corneg1, n(4137) corr(C)
>
> Henrique Neder
>
>
> -----Mensagem original-----
> De: owner-statalist@hsphsun2.harvard.edu
> [mailto:owner-statalist@hsphsun2.harvard.edu] Em nome de fjc
> Enviada em: terça-feira, 30 de agosto de 2011 23:00
> Para: statalist@hsphsun2.harvard.edu
> Assunto: st: generating a variable with pre-specified correlations with
> other two (given) variables
>
> Dear Statalisters:
>
> I have a dataset with two variables, x and y.
>
> I would like to generate a new artificial variable, z, with
> pre-specified correlations with x and y (no particular distribution
> required).
>
> Any help would be greatly appreciated.
>
> Best,
>
> Francisco.
>
> P.D. I'm using Stata 11 (on Windows XP)
> *
> *   For searches and help try:
> *   http://www.stata.com/help.cgi?search
> *   http://www.stata.com/support/statalist/faq
> *   http://www.ats.ucla.edu/stat/stata/
>
> -----
> Nenhum vírus encontrado nessa mensagem.
> Verificado por AVG - www.avgbrasil.com.br
> Versão: 10.0.1392 / Banco de dados de vírus: 1520/3868 - Data de Lançamento:
> 08/30/11
>
>
> *
> *   For searches and help try:
> *   http://www.stata.com/help.cgi?search
> *   http://www.stata.com/support/statalist/faq
> *   http://www.ats.ucla.edu/stat/stata/
>



-- 
Tirthankar Chakravarty
tchakravarty@ucsd.edu
tirthankar.chakravarty@gmail.com

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


© Copyright 1996–2014 StataCorp LP   |   Terms of use   |   Privacy   |   Contact us   |   Site index