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Re: st: re: heteroskedasticity and fixed effects

From   Maarten buis <>
Subject   Re: st: re: heteroskedasticity and fixed effects
Date   Thu, 17 Jul 2008 15:00:56 +0100 (BST)

--- aapdm <> wrote:
> The reason why I was asking about how to check for
> heteroskedasticity is the following. In a recent
> paper, Santos Silva and Tenreyro (2007) show that
> "under heteroskedasticity, the parameters of
> log-linearized models estimated by OLS lead to biased
> estimates of the true elasticities". They show that
> the Poisson estimator is more appropriate.
> I am estimating a model similar to a gravity equation
> of trade, where the dependent and independent
> variables are in logs and the data are time series
> cross-sectional. I wanted to check that
> heteroskedasticity is indeed present in order to
> justify the use of the Poisson estimator instead.

What about just looking at the residuals?

*--------------- begin example ------------------
webuse nlswork
generate age2 = age^2
generate ttl_exp2 = ttl_exp^2
generate tenure2 = tenure^2
generate byte black = race==2
xtset idcode

xtreg ln_w grade age* ttl_exp* tenure* black not_smsa south, fe
predict resid, e
predict yhat , xbu
twoway scatter resid yhat
*---------------- end example -------------------

-- Maarten

Maarten L. Buis
Department of Social Research Methodology
Vrije Universiteit Amsterdam
Boelelaan 1081
1081 HV Amsterdam
The Netherlands

visiting address:
Buitenveldertselaan 3 (Metropolitan), room Z434

+31 20 5986715

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