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From |
"Nick Cox" <n.j.cox@durham.ac.uk> |

To |
<statalist@hsphsun2.harvard.edu> |

Subject |
st: RE: Inflection point |

Date |
Wed, 14 Apr 2010 12:47:41 +0100 |

The turning point [not inflection point] of a quadratic is a property of the curve. Variability around the curve is immaterial, except for influencing how seriously we take the curve. However, although the quadratic you fitted will have a turning point somewhere; it need not be within the range of the data. If it is, your model seems a little implausible, at least on the information here. And a cubic polynomial with two turning points within the range of your data sounds even more implausible. Alternatively, it may that quadratic (and cubic) terms just capture nonlinearity in what is monotonic in the range of your data. Nick n.j.cox@durham.ac.uk Francesco Burchi I have run the following IV estimates: ivreg2 wage EDU EDUSquared X1 X2 (X3=Instrument1) X4 X5 X6 X7, robust I would like to calculate the inflection point of EDU (=years of schooling). Therefore, I have used the following formula: -b_EDU/(2*b_EDUSquared), Is this the right way to calculate the inflection point? I am surprised that I don't use the standard deviation in the formula. Moreover, is there a way to show graphically the effect of EDU controlling for all the other covariates (including the squared form and the endogenous variable X3) and visualizing the inflection point? Finally, should I use also the cubic form to see whether eventually there is more than one inflection point? I am using Stata 10. * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**References**:**st: Inflection point***From:*"Francesco Burchi" <fburchi@uniroma3.it>

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