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From |
Maarten buis <[email protected]> |

To |
[email protected] |

Subject |
Re: st: GB2 question |

Date |
Fri, 1 Jan 2010 12:57:21 +0000 (GMT) |

--- On Thu, 31/12/09, Jon Schwabish wrote: > Is there a way to fix the process so that I can estimate > the same predictions for the same people over multiple runs? > That is, when I run the following code it is not the case > that x1=x2 (even when setting a seed). > > gb2fit inc, stats > qgb2 inc, gen(x1) > > gb2fit inc, stats > qgb2 inc, gen(x2) > > Ideally, I'd like to generate the same predictions for each > person if I run my program multiple times. Do you get the same predictions if use -set sortseed- instead of -set seed-? -qgb2- looks at the quantile function, which usually implies sorting, and by default Stata sorts observations with exactly the same value randomly (from a logical point of view that is the most reasonable choice, but in practice it can lead to surprises like the one you are facing). Moreover, the random sorting is not governed by -set seed-, but by -set sortseed-. > Also, does anyone know how to construct a picture of the > pdf of the GB2 distribution? You can use -twoway function- to graph a function, like in the example below: *-------------------- begin example -------------------- local a = 1 local b = 2 local p = .5 local q = .25 local B = exp(lngamma(`p')+lngamma(`q')-lngamma(`p'+`q')) twoway function /// y = `a'*x^(`a'*`p'-1)*(`b'^(`a'*`p')) * /// `B'*(1 + (x/`b')^`a' )^(`p'+`q'))^-1, /// range(0 5) *----------------- end example -------------------------- Alternatively, if the aim is to compare your data with the Generalized Beta II distribution, you can use the -hangroot- package (If I am allowed some shameless self-promotion). This can be installed by typing -ssc install hangroot-, you can get an idea of the kind of graphs it produced by looking at: <http://www.maartenbuis.nl/software/hangroot.html> Hope this helps, Maarten -------------------------- Maarten L. Buis Institut fuer Soziologie Universitaet Tuebingen Wilhelmstrasse 36 72074 Tuebingen Germany http://www.maartenbuis.nl -------------------------- * * 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/

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