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Re: st: Shapes of two Inverted U curves (same model on two different samples)

From   Maarten Buis <>
Subject   Re: st: Shapes of two Inverted U curves (same model on two different samples)
Date   Tue, 5 Jul 2011 10:05:50 +0200

On Tue, Jul 5, 2011 at 9:51 AM, L.M.A. Mulotte  wrote me privately:
> I presume you mean I should use - suest - on each sample and then - test - on age2, right ?

Remember that the Statalist FAQ asks you not continue a discussion
outside the statalist. If you find my answer unclear, than chances are
that others following this discussion would feel the same way and
would thus also benefit from further clarification.

To answer your question, no, I mean you use the model with interaction
terms just as I advised you in my earlier post
<>. So, to
continue that example:

*-------------------------- begin example --------------------
use, clear
// estimate the model
reg ln_wage  i.south i.south#(c.age##c.age c.grade c.birth_yr)

// see how the coefficients are called
reg, coeflegend

// test whether both curves are equally peaked
// (that hypothesis is in this case rejected)
test _b[0b.south#c.age#c.age] = _b[1.south#c.age#c.age]
*-------------------- end example -------------------------
(For more on examples I sent to the Statalist see: )

Both -sureg- and the model with interaction terms have their
advantages and disadvantages, but I would stick to one of these models
and not test whether the turning points are equal within the model
with interactions and test whether the curves are equally steep within
the -sureg- model.

Hope this helps,

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