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From |
Mohammed El Faramawi <melfaram@yahoo.com> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: to Marteen buis (independent variable) |

Date |
Sun, 9 Mar 2008 07:00:56 -0700 (PDT) |

Thank you very much Maarten --- Maarten buis <maartenbuis@yahoo.co.uk> wrote: > If you transform the independent variable you are > changing the shape of > the relation between your depedent and independent > variable. You can > see some examples of such shapes by using the > -twoway function- graphs. > For instance, you asked about the log transformed x. > You can see such a > shape by typing: > > twoway function y = .1*ln(x), range(0 10) > > You will see that a unit increase in x has a > different impact depending > on where you start. A unit increase in x when you > start from x=.5 has a > much bigger impact on y than when you start from > x=8. > > Fox (1991) has a nice introduction on this issue in > term of linear > regression. > > -- Maarten > > John Fox (1991) Regression Diagnostics: An > Introduction. Thoasand Oaks: > Sage. > > > --- Mohammed El Faramawi <melfaram@yahoo.com> wrote: > > > Hi Marteen, > > You wrote Anyhow the real issue with transforming > the > > independent variables is whether the effect is > linear, > > or gradually slowing down (log) u-shaped, > quadratic, > > etc.' would you mind explaining more? especially > what > > do you mean by slowing down and (log) > > Thank you . > > --- Maarten buis <maartenbuis@yahoo.co.uk> wrote: > > > > > The independent variables don't need to be > normally > > > distributed. If you > > > log transfom the variable than you think that if > you > > > start with a small > > > dose, an increase has a much bigger effect than > when > > > you start with a > > > larger, which may or may not make sense. Anyhow > the > > > real issue with > > > transforming the indepedent variables is whether > the > > > effect is linear, > > > or gradually slowing down (log) u-shaped, > quadratic, > > > etc. As an aside, > > > if you compute percentile rank scores, you won't > > > make that variable > > > normal, uniformly distributed. > > > > > > -- Maarten > > > > > > --- Mohammed El Faramawi <melfaram@yahoo.com> > wrote: > > > > I have questions about the independent > variable > > > > transformation. > > > > I have an independent variable (lead mg/dl) > which > > > is > > > > not normally distributed. I am trying to run > it in > > > a > > > > cox hazard model the out come is mortality. > What > > > > should I do with lead > > > > 1) Should I log transform it? I did that but > it is > > > > still not normally ditributed after > transformatin. > > > > 2) Leave it as it is because transformation > does > > > not > > > > change the distribution? > > > > 3) categorize using quartiles or percentiles? > > > > > > > > > ----------------------------------------- > > > Maarten L. Buis > > > Department of Social Research Methodology > > > Vrije Universiteit Amsterdam > > > Boelelaan 1081 > > > 1081 HV Amsterdam > > > The Netherlands > > > > > > visiting address: > > > Buitenveldertselaan 3 (Metropolitan), room Z434 > > > > > > +31 20 5986715 > > > > > > http://home.fsw.vu.nl/m.buis/ > > > ----------------------------------------- > > > > > > > > > > > > > > > ___________________________________________________________ > > > > > > Rise to the challenge for Sport Relief with > Yahoo! > > > For Good > > > > > > http://uk.promotions.yahoo.com/forgood/ > > > * > > > * For searches and help try: > > > * > > > > http://www.stata.com/support/faqs/res/findit.html > > > * http://www.stata.com/support/statalist/faq > > > * http://www.ats.ucla.edu/stat/stata/ > > > > > > > > > > > > > > ____________________________________________________________________________________ > > Be a better friend, newshound, and > > know-it-all with Yahoo! Mobile. Try it now. > > > http://mobile.yahoo.com/;_ylt=Ahu06i62sR8HDtDypao8Wcj9tAcJ > > > > > * > > * For searches and help try: > > * > http://www.stata.com/support/faqs/res/findit.html > > * http://www.stata.com/support/statalist/faq > > * http://www.ats.ucla.edu/stat/stata/ > > > > > ----------------------------------------- > Maarten L. Buis > Department of Social Research Methodology > Vrije Universiteit Amsterdam > Boelelaan 1081 > 1081 HV Amsterdam > The Netherlands > > visiting address: > Buitenveldertselaan 3 (Metropolitan), room Z434 > > +31 20 5986715 > > http://home.fsw.vu.nl/m.buis/ > ----------------------------------------- > > > > ___________________________________________________________ > > Rise to the challenge for Sport Relief with Yahoo! > For Good > > http://uk.promotions.yahoo.com/forgood/ > * > * For searches and help try: > * > http://www.stata.com/support/faqs/res/findit.html > * http://www.stata.com/support/statalist/faq > * http://www.ats.ucla.edu/stat/stata/ > ____________________________________________________________________________________ Never miss a thing. Make Yahoo your home page. http://www.yahoo.com/r/hs * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**References**:**Re: st: to Marteen buis (independent variable)***From:*Maarten buis <maartenbuis@yahoo.co.uk>

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