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From |
"Tim Wade" <wadetj@gmail.com> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: Help Pls! st: fracpol basic question |

Date |
Tue, 18 Dec 2007 13:36:21 -0500 |

mai mai, references for fracpoly cited in the stata manual are: Royston and Altman 1994 Applied Statistics 43: 429-467 as well as several references in the Stata Bulletin. Maybe these can help you. If you do a search on fractional polynomial regression in pubmed or google scholar you will find other relevant publications. I do not consider it "data mining", but an approach to explore non linear relationships. Good luck, Tim On 12/18/07, mai mai <mai7777@gmail.com> wrote: > Hi, > I have couple of questions on Fracpol, I tried finding answers through > help and the internet but was unsuccessful. The results of my model are > posted below. > > 1) How acceptable is the use of fractional polynomial in the > literature. Is it considered data mining? > > 2) What is the structure of the best relation? > Is it: y=b0 + b1 X^-.5 + X^-.5*ln(X) ? > > 3) Why is X = (dist+2.860107421875)/1000)? why the addition of 2.8 > and division by 1000? > > 4) What is the exact meaning of deviance? Is lower (more negative) > better or worst? > > . fracpoly reg b dist, adjust (no) compare > ........ > -> gen double Idist__1 = X^-.5 if e(sample) > -> gen double Idist__2 = X^-.5*ln(X) if e(sample) > (where: X = (dist+2.860107421875)/1000) > > Source | SS df MS Number of obs = 57 > -------------+------------------------------ F( 2, 54) = 3.45 > Model | .014450698 2 .007225349 Prob > F = 0.0390 > Residual | .113204826 54 .002096386 R-squared = 0.1132 > -------------+------------------------------ Adj R-squared = 0.0804 > Total | .127655524 56 .002279563 Root MSE = .04579 > > ------------------------------------------------------------------------------ > y | Coef. Std. Err. t P>|t| [95% Conf. Interval] > -------------+---------------------------------------------------------------- > Idist__1 | .04901 .0190668 2.57 0.013 .0107834 .0872366 > Idist__2 | .007933 .0031175 2.54 0.014 .0016829 .0141832 > _cons | -.0443816 .0171458 -2.59 0.012 -.0787569 -.0100063 > ------------------------------------------------------------------------------ > Deviance: -192.87. Best powers of distance among 44 models fit: -.5 -.5. > > Fractional polynomial model comparisons: > ------------------------------------------------------------------------------ > dist df Deviance Res. SD Dev. dif. P [*] Powers > ------------------------------------------------------------------------------ > Not in model 0 -186.025 .047745 6.848 0.174 > Linear 1 -188.380 .047192 4.492 0.247 1 > m = 1 2 -189.268 .046825 3.604 0.193 .5 > m = 2 4 -192.873 .045786 -- -- -.5 -.5 > ------------------------------------------------------------------------------ > [*] P-value from deviance difference comparing reported model with m = 2 model > > > Many thanks for your help > * > * 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/ > > * > * 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/ > * * 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**:**Help Pls! st: fracpol basic question***From:*mai mai <mai7777@gmail.com>

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