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From |
Fabio Zona <fabio.zona@unibocconi.it> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: linear, quadratic terms and centering |

Date |
Sat, 21 Aug 2010 11:20:22 +0200 (CEST) |

Thank you very much Michael! One more information: I have to make a complex interaction, whereby a linear and a square term are both interacted with a linear (continuous) variable x2: y = a + x1 + x1_Square + x2 + x2 x1 + x2 x1_Square One of my VIF ( the one related to x2 x1_Square ) is just above 10 ( it is 10,949), while the maximum condition index reaches the value of 19,782 (for the same interaction term x2 x1_Square ). Do I have a problem of multicollinearity with this value just above the threshold of 10 ? How can I fix this problem ? I know there is this command in STATA orthog; how does this command work? I though that I had to calculate the quadratic term and then use orthog between the linear and the quadratic term. However, since you said that (for centering variables) I first center the linear term and then calculate the quadratic term, I get confused also about orthog! Thanks ----- Messaggio originale ----- Da: "Michael N. Mitchell" <Michael.Norman.Mitchell@gmail.com> A: statalist@hsphsun2.harvard.edu Inviato: Sabato, 21 agosto 2010 10:43:12 GMT +01:00 Amsterdam/Berlino/Berna/Roma/Stoccolma/Vienna Oggetto: Re: st: linear, quadratic terms and centering Dear Fabio You were correct when you wrote... Or do I first center the linear term and calculate the square term on the basis of the "centered" linear term ? Or, in Stata, you can do this (say you want to center -x- around 100)... generate xcentered = x - 100 regress y c.xccentered##c.xccentered That -regress- command will include -xcentered- as well as the squared term. Hope this helps, Michael N. Mitchell Data Management Using Stata - http://www.stata.com/bookstore/dmus.html A Visual Guide to Stata Graphics - http://www.stata.com/bookstore/vgsg.html Stata tidbit of the week - http://www.MichaelNormanMitchell.com On 2010-08-21 12.40 AM, Fabio Zona wrote: > Dear Statalist, > > a very simple question: I have > > y = c + x0 + x1 + x1square > > In order to center x1 and x1square, do I first need to calculate the square term and afterwards to center both the linear and the quadratic term around their respective means? > Or do I first center the linear term and calculate the square term on the basis of the "centered" linear term ? > > I guess that the first alternative works. > > Thanks > > * > * 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/ * * 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/ * * 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/

**Follow-Ups**:**Re: st: linear, quadratic terms and centering***From:*"Michael N. Mitchell" <Michael.Norman.Mitchell@gmail.com>

**References**:**Re: st: linear, quadratic terms and centering***From:*"Michael N. Mitchell" <Michael.Norman.Mitchell@gmail.com>

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