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Re: st: Polynomial OLS - Interpreting results

From   Maarten buis <>
Subject   Re: st: Polynomial OLS - Interpreting results
Date   Tue, 30 Dec 2008 19:35:34 +0000 (GMT)

--- Claude Francoeur <> wrote:
> We were expecting to have 2 bends in our model (y = X1 X1squared 
> X1cubed). <snip> How should we interpret these results?

The easiest way to interpret a non-linear regression curve is to graph
the predicted value against the original explanatory variable. -adjust-
can be quite handy here, as it allows you to fix the other explanatory
variables at certain values (and can create confidence intervals around
the predicted values).

*------------- begin example ------------------
sysuse auto, clear
sum weight
gen z_weight = (weight-r(mean))/r(sd)
gen z_weight2 = z_weight^2
gen z_weight3 = z_weight^3
reg mpg z_* foreign price

adjust foreign = 0 price, by(weight) ci replace

twoway rarea lb ub weight, sort || ///
       line xb weight, legend(off) sort
*--------------- end example --------------------
(For more on how to use examples I sent to the Statalist, see )

Maarten L. Buis
Department of Social Research Methodology
Vrije Universiteit Amsterdam
Boelelaan 1081
1081 HV Amsterdam
The Netherlands

visiting address:
Buitenveldertselaan 3 (Metropolitan), room N515

+31 20 5986715

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