It's the latter. You can think of matching in observational studies as a
way to have the treatment and control groups that have similar covariate
distributions. From this perspective, it does not matter which units are
matched with each other. What matters is the resulting treatment and
control goups that have good covariate balance. Of course, if you do
subclassification, then the model will be estimated within each subclass.
Kosuke
--
Department of Politics
Princeton University
http://imai.princeton.edu
On Sat, 13 Jun 2009, Prashant wrote:
Hi,
I'm new to Zelig. I was trying to figure out how Zelig simulates the
counterfactuals for a treatment group (after already having run matchit to
get treatment and control groups) to estimate the ATT. I think in basic
terms it estimates the parameters of a model (that you specify) for the
matched control group and then uses the estimated parameters and treatment
group's covariates to estimate predicted/expected values--these are the
counterfactuals for the treatment group.
But here's my question (and it is probably due to a lack of conceptual
understanding): Sometimes matching is in pairs (for example)--does the
simulation also take account for this? That is, if persons A, B, C in the
treatment group are matched to persons D, E, F respectively in the control
group, does the simulation take into account that matching has grouped A &
D, B & E, C & F? Or does the simulation just take the mass of the matched
control group observations and estimate a model through it to create
estimated parameters (to be used with the treatment group's covariates to
create counterfactuals)?
Sorry in advance if the question is not clear.
best,
Prashant
-
Zelig Mailing List, served by Harvard-MIT Data Center
Send messages: zelig(a)lists.gking.harvard.edu
[un]subscribe Options:
http://lists.gking.harvard.edu/?info=zelig
Zelig program information:
http://gking.harvard.edu/zelig/