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From |
David Hoaglin <dchoaglin@gmail.com> |

To |
statalist@hsphsun2.harvard.edu |

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

Date |
Fri, 26 Apr 2013 17:35:38 -0400 |

Gabriel, I second Nick's advice to abandon -ladder-. Choosing a transformation involves a fair amount of judgment, and I would not delegate the choice to an automated process. I also have some other comments. The number of people who reported being displaced by violence is a count. Sometimes the square root is a reasonable transformation for counts, but large counts often need a logarithm. As Nick suggested, however, a Poisson model may be appropriate or perhaps a negative binomial model. Before I tried such models, however, I would want to know why your data did not include any zeros. Is 1010 the total number of municipalities in Colombia, or do your data include only municipalities in which at least 1 person reported being displaced? Either a Poisson distribution or a negative binomial distribution would have positive probability of producing some zeros. If zeros have been excluded, the model would have to handle that feature. Another consideration, perhaps important, is that the usual Poisson and negative binomial models assume that the occurrences are independent. The nature of your data suggests that some types of clustering are likely to be involved. An episode of violence is likely to cause a number of people to be displaced simultaneously, and it might affect nearby municipalities similarly. Yet another feature of the data is the size of the municipality. The number of people displaced might be related to the population of the municipality. Do you have data on the populations? You said that the data do not show bimodal structure, but I could easily imagine that they represent a mixture of distributions, maybe having several components. Do you have other variables that might help to account for structure in the data (geographic and otherwise)? I am probably making your analysis more complicated, but I hope I am making it more realistic. David Hoaglin On Fri, Apr 26, 2013 at 4:57 PM, Gabriel Nelson <lgabrielnelson@gmail.com> wrote: > Thanks very much for your suggestions Nick. It makes sense that the > problem might lie within -sktest-. I won't worry any more about this > problem and just proceed with the qnorm command, as you suggested. > Thanks again. > > Gabriel * * 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:*Gabriel Nelson <lgabrielnelson@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>

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

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

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