Notice: On April 23, 2014, Statalist moved from an email list to a forum, based at statalist.org.
[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: st: ladder question for right-skewed variable
From
Nick Cox <[email protected]>
To
"[email protected]" <[email protected]>
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
[email protected]
On 26 April 2013 18:55, Gabriel Nelson <[email protected]> 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 <[email protected]> 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/