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Re: st: xtabond2 - variables in level
Hi Statalisters,
one users already gave me an answer which was not what I was looking for
because I stated my problem in the wrong way. I will try to explain my
problem more carefully.
Suppose I want to estimate a basic Cobb Douglas production function.
Please ignore mispecification of the model or endogeneity issues, since
here I will keep the model oversimplified on purpose. The model I want
to estimate is the following:
y_it = B1*y_i(t-1) + B2*k_it + B3*l_it + B4*w_it + u_i + e_it
where y is log of added value, k and l are logs of capital and labor and
w represents unobservable total factor productivity (TFP) and u_i is the
fixed effect. Now, if I take first differences, I would obtain:
D(y_it)= B1*D(y_i(t-1)) + B2*D(k_it) + B3*D(l_it) + B4*D(w_it) + D(e_it).
where D(...) means first difference. Being the TFP unobservable, one has
to proxy for it. I will proxy for it by taking a variable called /rents/
lagged one period, which is a firm-level measure of competition.
However, I want to get a coefficient for the level of rents rather than
its first difference. The model I would want to estimate should be
D(y_it)= B_t + B1*D(y_i(t-1)) + B2*D(k_it) + B3*D(l_it) +
B4*rents_i(t-1) + D(e_it).
The reference papers I am using estimated this function using DPD98 for
Gauss. However, I want to find out whether it is possible to estimate
that function using xtabond2. In Stata, I would write
xi: xtabond2 y yL1 k l rentsL1 i.year, gmm(yL1) iv(i.year k l rentsL1)
noleveleq small
where all the variable are in logs, and *L1 means one period lag. If I
am not wrong, if I do so the coefficient I get for rentsL1 will be on
its first difference (i.e. rentsL1 - rentsL2). Is there a way to have
the coefficient for level of rents, keeping all the other variables in
first difference?
I hope someone is able to help me out!
Tomasz Boniek
Tomasz Boniek wrote:
Hi,
I have a problem with xtabond2. Suppose I run the following:
xtabond2 ln_tfp L.ln_tfp L.rents yr*, gmm(L.ln_tfp L.rents, lag(2 3))
iv(yr*) robust noleveleq small
where yr* are a series of year dummies and the economic meaning of the
other variable is irrelevant for my question.
In this case, if I'm not wrong, I will estimate a one-step difference
GMM. All the variables in this case will be in first difference. My
question is:
Is there a way to avoid differentiation of the variable L.rents? I
would like to use as a regressors (treating it as endogenous) not the
first difference lagged one period (i.e. rents(t-1)-rents(t-2)) but
just the lagged level (i.e. rents(t-1)). Is there a way to do that in
Stata?
Thanks for the help,
Tomasz Boniek
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