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# Re: st: Re: joint effect of two endogenous variables

 From Maarten buis To statalist@hsphsun2.harvard.edu Subject Re: st: Re: joint effect of two endogenous variables Date Tue, 24 Aug 2010 16:05:52 +0000 (GMT)

```--- On Tue, 24/8/10, xueliansharon wrote:
> my y1 is people's age started to work, y2 is years of schooling,
> what I want to get is the joint effect when people started to
> enter the labor market one year later and receive one more year of
> schooling.

It sounds like the unit of both variables is years, but they are
obviously still different, a year of education is not the same
thing as getting a year older, so you'll still need to standardize.

> But I didn't understand why "constraining the effects to be
> equal" could compute the joint effect, could you explain it more
> explicitly?

You'll need to conceptually define what a "joint effect" is, and
you could see constraining the effects to be equal as one such
definition.

By proposing one such definition, I side steped the most important
part: thinking about what it is that you want to measure. So
instead of focussing on technique, I would first try to find a
reason why one might want to know the effect of the combination of
these two variables. If you can come up with such a story, then
you can often derive the type of constraint that you need to apply.
If you cannot come up with such a story, then estimating a "joint
effect" just doesn't make sense, and you'll need to find another
solution.

Hope this helps,
Maarten

--------------------------
Maarten L. Buis
Institut fuer Soziologie
Universitaet Tuebingen
Wilhelmstrasse 36
72074 Tuebingen
Germany

http://www.maartenbuis.nl
--------------------------

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