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From |
Nick Cox <njcoxstata@gmail.com> |

To |
"statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu> |

Subject |
Re: st: ladder question for right-skewed variable |

Date |
Fri, 26 Apr 2013 19:17:48 +0100 |

That's not quite "no transformations appeared in the output" as -ladder- is signalling P-values for some cases. But I readily agree that -ladder- is not doing a good job here at all. In fact, I am now reminded of evident -ladder- problems shown in a recent thread starting at http://www.stata.com/statalist/archive/2013-02/msg00862.html I can't find a public email, even though I thought I posted on this, but my impression from looking at the code is that -ladder- is essentially fragile. The real problem here is within -sktest-. It can break down, it seems, for large sample sizes and/or large deviations from Gaussianity. Then it bounces back missings. I think you just need to abandon -ladder-. It's not essential. You don't need _any_ test to tell you that some transformation will help if the goal is to reduce asymmetry, and there are only a few credible alternatives. As David and I pointed out, log transformation should work quite well for your data, but but but: (my suggestion; David may not agree) why transform at all? Your solutions start with -poisson- (or, for consenting adults, -nbreg-). BTW, -ladder- is a command, not a function, and in Stata ne'er the twain shall meet. Nick njcoxstata@gmail.com On 26 April 2013 18:55, Gabriel Nelson <lgabrielnelson@gmail.com> wrote: > Thanks Nick, yes exactly, my question is why the ladder function fails > to provide any chi-square values here. I'll attach the Stata output > here: > > . ladder disp_2000 > > Transformation formula chi2(2) P(chi2) > ------------------------------------------------------------------ > cubic dis~2000^3 . . > square dis~2000^2 . . > identity dis~2000 . . > square root sqrt(dis~2000) . 0.000 > log log(dis~2000) . 0.000 > 1/(square root) 1/sqrt(dis~2000) . 0.000 > inverse 1/dis~2000 . 0.000 > 1/square 1/(dis~2000^2) . 0.000 > 1/cubic 1/(dis~2000^3) . 0.000 > > . sum disp_2000, detail > > Number displaced 2000 (if data unavailable go up > to 2003 > ------------------------------------------------------------- > Percentiles Smallest > 1% 1 1 > 5% 2 1 > 10% 3 1 Obs 1010 > 25% 6 1 Sum of Wgt. 1010 > > 50% 15.5 Mean 281.5297 > Largest Std. Dev. 1217.168 > 75% 82 9421 > 90% 436.5 9505 Variance 1481497 > 95% 1251 16255 Skewness 9.012044 > 99% 5953 19569 Kurtosis 108.8061 > > On Fri, Apr 26, 2013 at 10:47 AM, Nick Cox <njcoxstata@gmail.com> wrote: >> Please see my answers too. You have still not given the exact -ladder- >> command you used or its output, so it is really difficult to know what >> is going on. * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/faqs/resources/statalist-faq/ * http://www.ats.ucla.edu/stat/stata/

**Follow-Ups**:**Re: st: ladder question for right-skewed variable***From:*Nick Cox <njcoxstata@gmail.com>

**References**:**st: ladder question for right-skewed variable***From:*Gabriel Nelson <lgabrielnelson@gmail.com>

**Re: st: ladder question for right-skewed variable***From:*David Hoaglin <dchoaglin@gmail.com>

**Re: st: ladder question for right-skewed variable***From:*Gabriel Nelson <lgabrielnelson@gmail.com>

**Re: st: ladder question for right-skewed variable***From:*Nick Cox <njcoxstata@gmail.com>

**Re: st: ladder question for right-skewed variable***From:*Gabriel Nelson <lgabrielnelson@gmail.com>

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