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

From   Maarten buis <>
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 

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 

Hope this helps,

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


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