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RE: st: calculating effect sizes when using svy command
From
"Vogt, Dawne" <[email protected]>
To
"'[email protected]'" <[email protected]>
Subject
RE: st: calculating effect sizes when using svy command
Date
Wed, 8 Dec 2010 10:54:02 -0500
Thanks to everyone who weighed in on this issue. I figured out how to compute the weighted partial correlations with aweights. I noticed, however, that the p values associated with these weighted partial correlations are different than the p values associated with the t values that are presented in the svyreg output. I understand that the relationship between partial rs and t values only hold for ordinary least squares, but I am having trouble reconciling cases where the p values associated with the t for a particular predictor is not significant (p <.05) but the p value associated with the partial r for the same variable is significant. I'm not sure what to make of it. Do people have thoughts or suggestions?
-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Steven Samuels
Sent: Wednesday, December 08, 2010 8:56 AM
To: [email protected]
Subject: Re: st: calculating effect sizes when using svy command
--
The simplest way of computing the weighted partial correlation is to
use -pcorr- with aweights, as suggested by Bill Sribney's FAQ. In the
example, repeated below, the difference is R-square from regression of
residuals = 0.259; partial correlation-squared from -pcorr- = 0.288.
Steve
**************************CODE BEGINS**************************
sysuse auto, clear
svyset rep78 [pw=trunk]
/* Compute partial correlation of mpg and weight, controlling for turn
*/
svy: reg mpg weight turn
svy: reg mpg turn
predict r_mt, resid
svy: reg weight turn
predict r_wt, resid
svy: reg r_mt r_wt //R-squared is partial r-square
pcorr mpg weight turn [aw=trunk]
***************************CODE ENDS***************************
On Dec 7, 2010, at 5:39 PM, Steven Samuels wrote:
Well, I got the details wrong, but the example right.
If the multiple regression is:
svy: reg y x z
svy: reg y z, with residual r_yz
svy: reg x z, with residual r_xz
Thecorrelation of r_yz and r_xz is the partial correlation r_yx.z So
the goal is to estimate the correlation of r_yz and r_xz with one of
the methods in the post, including
svy: reg r_yz r_xz
Sorry for the confusion.
Steve
[email protected]
On Dec 7, 2010, at 5:03 PM, Steven Samuels wrote:
--
What Dawne called "correlation" is actually the partial correlation
of y and x, controlling for other covariates z (r_yx.z). The partial
correlation in OLS can be estimated by t^2/(df_error + t^2), but that
is not true for survey regression. However it can be estimated in a
three step process: -svy regress- y on x, with residual r_yx ; y on
the z's with residual r_yz. Then compute the correlation of r_yx and
r_yz by means of 1) Nick's program 2) the methods in Bill Sribney's
FAQ; or 3) by running -svy: reg r_yx r_yz- and taking the R-squared
reported by that command. The p-value issue discussed by Bill doesn't
arise for Dawn, because she takes the p-value from the original
regression. See below for an example.
Steve
**************************CODE BEGINS**************************
sysuse auto, clear
svyset rep78 [pw=trunk]
/* Compute partial correlation of mpg and weight, controlling for turn
*/
svy: reg mpg weight turn
svy: reg mpg turn
predict r_mt, resid
svy: reg weight turn
predict r_wt, resid
svy: reg r_mt r_wt //R-squared is partial r-square
***************************CODE ENDS***************************
On Dec 7, 2010, at 4:18 PM, Nick Winter wrote:
Re: svy and (bivariate) correlation, this FAQ talks about how to do
the equivalent of the nonexistent -svy: correlate-:
http://www.stata.com/support/faqs/stat/survey.html
The short version is that the point estimate is -correlate- with
aweights, and the p-value as you discuss below.
My -corr_svy- from SSC implements this approach, though it is a Stata
version 7 program so it does not take advantage of Stata's current,
more extensive -svy- features.
-Nick Winter
On 12/7/2010 4:11 PM, Vogt, Dawne wrote:
> Thanks. So it sounds like I can take the square root of the R
> squared value to get the correlation coefficient for a regression
> with 1 predictor. But how do I get effect size indicators
> (preferably in the form of correlation coefficients) for each
> predictor in a regression with multiple predictors?
>
>
> -----Original Message-----
> From: [email protected] [mailto:[email protected]
> ] On Behalf Of Steven Samuels
> Sent: Tuesday, December 07, 2010 4:00 PM
> To: [email protected]
> Subject: Re: st: calculating effect sizes when using svy command
>
> --
> -
> I should have added: The relation of (partial) r-squares to t-
> statistics holds only for ordinary least squares, not for the
> estimation formulas of survey regression. So, neither of your
> calculated r's is correct.
>
> Steve
>
> On Dec 7, 2010, at 3:31 PM, Vogt, Dawne wrote:
>
> I have two questions related to calculating effect sizes using svyreg
> (pweights):
>
> First, when doing unweighted regressions in SPSS, I like to provide
> effect sizes for each predictor by calculating a correlation
> coefficient value (r) from the t values provided in the output. I like
> using r because it is easy for most people to interpret. Can I do the
> same using svyreg output?
>
> My second question is related to the first. Since there is no
> correlation option under the svy commands, I have been computing
> regressions of Y on X and X and Y and using the largest p value of the
> two sets of results, as recommended elsewhere. I've having trouble
> figuring out how to convert the results provided in the output to a
> correlation coefficient though. I noticed that the r value I get by
> taking the square root of the R squared is different from my own hand
> calculation of r derived from the t value provided in the regression
> output [sqrt of (t squared divided by t squared + df). I'm not sure
> which r is correct (or if either of them are correct).
>
> Thanks in advance for any guidance others may be able to offer.
>
>
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--
--------------------------------------------------------------
Nicholas Winter 434.924.6994 t
Assistant Professor 434.924.3359 f
Department of Politics [email protected] e
University of Virginia faculty.virginia.edu/nwinter w
S385 Gibson Hall, South Lawn
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