Jim - One way is after the initial -drawnorm-, to sort by subject, then
replace u by its first value within each subject:
sort subject
by subject:replace u=u[1]
After this, you can implement what I have sent previously. If you want
the mean to be exactly zero, just subtract the sample mean after the
above.
Of course, it is up to you to interpret what it means for u to be
correlated with X1 when u is constant within a subject and X1 is time
varying. There are clearly many different types of models that you could
use to generate this correlation. The one I suggested above is one of
them.
Al
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of James Shaw
Sent: Tuesday, December 23, 2008 12:58 PM
To: [email protected]
Subject: Re: st: RE: Simulating multilevel data in Stata
Thanks for your prompt response. The code you sent does a great job at
producing the desired correlations. The only problem is that it
produces a variable u that is time-varying. The variable u is meant to
represent a random effect. As such, it should have a mean of 0 and vary
by subject only (i.e., be time-invariant). The variables X1 and
X2 should vary by subject (i) and time (t). The final product needs to
be a time-invariant u that is correlated 0.5 with time-varying X1 and
0.0 with time-varying X2. Is there a way to modify the code to get the
desired correlations?
--
Jim
On Tue, Dec 23, 2008 at 11:15 AM, Feiveson, Alan H. (JSC-SK311)
<[email protected]> wrote:
> Jim -
>
> If I understand your question, you want to "fix" your randomly
> generated X1, X2 and u so that their sample covariance matrix exactly
> equals the one you want. Here's one way to do this (see below.
>
> Al Feiveson
>
>
> . matrix V = I(3)
> . matrix V[1,2]=.5
> . matrix V[2,1]=.5
> . matrix V[1,3]=.3
> . matrix V[3,1]=.3
>
> . matrix list V
>
> symmetric V[3,3]
> c1 c2 c3
> r1 1
> r2 .5 1
> r3 .3 0 1
>
> . set obs 50
> obs was 0, now 50
>
> . drawnorm X1 X2 u,cov(V)
>
> . corr X1 X2 u,cov
> (obs=50)
>
> | X1 X2 u
> -------------+---------------------------
> X1 | 1.01209
> X2 | .467516 .953286
> u | .571851 .073683 1.12376
>
> . matrix accum A = X1 X2 u,dev noc
> (obs=50)
>
> . matrix A=(1/49)*A
>
> . matrix list A
>
> symmetric A[3,3]
> X1 X2 u
> X1 1.0120898
> X2 .46751642 .95328608
> u .5718511 .07368264 1.123759
>
> . matrix H=cholesky(V)
> . matrix G=cholesky(A)
> . matrix GI=inv(G)
>
> . matrix HGI=H*GI
>
> . matrix list HGI
>
> HGI[3,3]
> X1 X2 u
> r1 .99400935 0 0
> r2 .03111956 1.0085584 0
> r3 -.34919894 .07784363 1.082162
>
> . des
>
> Contains data
> obs: 50
> vars: 3
> size: 800 (99.9% of memory free)
> ----------------------------------------------------------------------
> --
> ---------------------------
> storage display value
> variable name type format label variable label
> ----------------------------------------------------------------------
> --
> ---------------------------
> X1 float %9.0g
> X2 float %9.0g
> u float %9.0g
> ----------------------------------------------------------------------
> --
> ---------------------------
> Sorted by:
> Note: dataset has changed since last saved
>
> . gen y1=HGI[1,1]*X1
> . gen y2=HGI[2,1]*X1+HGI[2,2]*X2
> . gen y3=HGI[3,1]*X1+HGI[3,2]*X2+HGI[3,3]*u
>
> . corr y*,cov
> (obs=50)
>
> | y1 y2 y3
> -------------+---------------------------
> y1 | 1
> y2 | .5 1
> y3 | .3 -9.1e-10 1
>
>
>
>
> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]] On Behalf Of James Shaw
> Sent: Tuesday, December 23, 2008 10:55 AM
> To: [email protected]
> Subject: st: Simulating multilevel data in Stata
>
> Dear Statalist members,
>
> I want to perform a simulation to show the inconsistency of the OLS
> and random effects estimators when one of the regressors is correlated
> with the unit-specific error component. The specifics of the
> simulation are as follows;
>
> Y[i,t] (the outcome to be modeled) = b0 + b1*X1[i,t] + b2*X2[i,t] +
> u[i]
> + e[i,t]
>
> i = 1,...,500 indexes subjects
> t = 1,...,3 indexes time (repeated observations on subjects)
>
> X1 and X2 are normally distributed random variables with arbitrary
> means and variances u is a normally distributed subject-specific error
> component with mean of 0 and arbitrary variance e is a normally
> distributed random error component with mean of 0 and arbitrary
> variance
>
> corr(X1,X2) = 0.5
> corr(X1,u) = 0.3
> corr(X2,u) = 0.0
> corr(X1,e) = corr(X2,e) = corr(u,e) = 0.0
>
> b0, b1, and b2 are parameters to be specified in the simulation
>
>
> I have been unable to identify a method that will ensure that
> corr(X1,u) equals the desired value. I tried the following method in
> which u was generated separately from X1 and X2 and cholesky
> decomposition was applied to generate transformations of the three
> random variables that would exhibit the desired correlations.
> However, this yielded a non-zero correlation between X2 and u.
>
> Method 1
> ***
> drop _all
> set obs 500
> gen n = _n
> gen u=invnorm(uniform())
> expand 3
> sort n
> gen n2 = _n
> gen t= (n2 - (n-1)*3)
> drawnorm x1 x2 e
> sort n
> mkmat x1 x2 u e, matrix(X)
> mat c =(1, .5, .3, 0 \ .5, 1, 0, 0 \ .3, 0, 1, 0 \ 0, 0, 0, 1) mat X2
> =
> X*cholesky(c)
> ***
>
> A method that yielded somewhat better results involved generating X1,
> X2, u, and e with a pre-specified correlation matrix and then
> collapsing u so that it varied by subject only. This provided the
> correct values for corr(X1,X2) and corr(X2,u) but attenuated the
> correlation between X1 and u. I presume that I could simply specify a
> higher value for
> corr(X1,u) when generating the variables so that the desired value
> would be achieved after u is collapsed. However, this would not be
> the most elegant solution.
>
> Method 2
> ***
> drop _all
> set obs 500
> mat c =(1, .5, .3, 0 \ .5, 1, 0, 0 \ .3, 0, 1, 0 \ 0, 0, 0, 1) gen n =
> _n expand 3 sort n gen n2 = _n gen t= (n2 - (n-1)*3) drawnorm x1 x2 u
> e,
> corr(c) sort n by n: egen u2 = mean(u)
> ***
>
> Any suggestions or references would be appreciated.
>
> Regards,
>
> Jim
>
>
> James W. Shaw, Ph.D., Pharm.D., M.P.H.
> Assistant Professor
> Department of Pharmacy Administration
> College of Pharmacy
> University of Illinois at Chicago
> 833 South Wood Street, M/C 871, Room 252 Chicago, IL 60612
> Tel.: 312-355-5666
> Fax: 312-996-0868
> Mobile Tel.: 215-852-3045
> *
> * 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/
>
> *
> * 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/
>
--
James W. Shaw, Ph.D., Pharm.D., M.P.H.
Assistant Professor
Department of Pharmacy Administration
College of Pharmacy
University of Illinois at Chicago
833 South Wood Street, M/C 871, Room 252 Chicago, IL 60612
Tel.: 312-355-5666
Fax: 312-996-0868
Mobile Tel.: 215-852-3045
*
* 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/
*
* 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/
