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From |
Maarten buis <maartenbuis@yahoo.co.uk> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: Frapoly |

Date |
Mon, 11 Feb 2008 09:46:24 +0000 (GMT) |

--- mai mai <mai7777@gmail.com> wrote: > Is there a why to find out the inflection points, trough and top of > the fracpoly or fracplot graph? You are basically asking when the first and second derivative are zero. I find a graph of the first or second derivative with confidence intervals against your explanatory variable instructive as it also gives some sense of the precision of the estimate. These are pretty subtle features of the curve so you can expect quite a bit of uncertainty which just a point estimate tends to hide. -fracpoly- fits a polynomial, so the first and second derivatives are pretty easy to derive (though as I am lazy and sloppy I find a page like this invaluable: http://www.quickmath.com/ ). Once you have derived the first and second derivative you can use -predictnl- to get predictions of the first and second derivative with their confidence intervals, which you can than graph. In the example below I illustrate this procedure with the first derivative. Hope this helps, Maarten *------------------- begin example --------------------- sysuse nlsw88, clear gen ln_w = ln(wage) fracpoly reg ln_w ttl_exp return list local x "(`e(fp_x1)'-`e(fp_shft)')/`e(fp_sfac)'" predictnl dydx = _b[Ittl___1]*2*`x' + /// _b[Ittl___2]*(2*ln(`x')*`x' + `x'), /// ci(lb ub) sort ttl_exp twoway rarea lb ub ttl_exp || /// line dydx ttl_exp, /// yline(0) legend(off) *--------------------------- end example -------------------- (For more on how to use examples I sent to the Statalist, see http://home.fsw.vu.nl/m.buis/stata/exampleFAQ.html ) ----------------------------------------- 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/ ----------------------------------------- __________________________________________________________ Sent from Yahoo! Mail - a smarter inbox http://uk.mail.yahoo.com * * 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**:**st: Frapoly***From:*"mai mai" <mai7777@gmail.com>

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