sas, Log and other transforms (was How to back transform

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sas >> Log and other transforms (was How to back transform

by davidlcassell » Thu, 29 Mar 2007 08:14:24 GMT

XXXX@XXXXX.COM replied:
>
> >>> David L Cassell < XXXX@XXXXX.COM > 03/28/07 12:56 AM >>> wrote
><<<
>In my experience, log-transforming to correct for skewness was usually
>followed by disgruntled grumbling, as the log transform corrected too
>much or not enough, or ruined some other aspect of the underlying
>assumptions. I haven't had a real data set which called out for a log
>transform since the 1980's.
> >>>>
>
>
>I agree with David that blindly transforming things because 'that's what
>people do' is silly and likely to be
>counter-productive.
>
>I think a better approach is to figure out whether the transformation
>makes any substantive sense. If the scale is arbitrary and not
>well-known, and you are lucky enough that some transformation creates
>fewer problems than it solves, then it may be sensible to use the BoxCox
>transformation, even if the transformation is bizarre. OTOH, if the
>scale is known, or inherently sensible, then transformation is less
>likely to be the ideal solution, and, if a transformation is necessary,
>it should be one that preserves the sense of the variable.
>
>So......a colleague wanted to log transform a ratio. WAIT, I said.
>Does it make any substantive sense to change a ratio into a difference?
>(It did not). OTOH, if called for, an inverse transformation of a ratio
>can make sense (changing miles per gallon to gallons per mile)
>
>As always, I think the substantive question should drive the statistics,
>not vice versa
>
>
>HTH
>
>Peter

I agree.


And also, watch out for those Box-Cox lambdas. I had a project once
where some people wanted to use Box-Cox for transformations to
normality, and they were getting typical values like 2.3836 and 0.183733
and they weren't even thinking that the first was like a square and
the second was like a log transform. (Not that any of the transforms
were actually appropriate for what they wanted to do with the data, but
that's another issue.)

David
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1. Log and other transforms (was How to back transform a 

>>> David L Cassell < XXXX@XXXXX.COM > 03/28/07 12:56 AM >>> wrote
<<<
In my experience, log-transforming to correct for skewness was usually
followed by disgruntled grumbling, as the log transform corrected too
much or not enough, or ruined some other aspect of the underlying
assumptions.  I haven't had a real data set which called out for a log
transform since the 1980's.
>>>>


I agree with David that blindly transforming things because 'that's what
people do' is silly and likely to be
counter-productive.

I think a better approach is to figure out whether the transformation
makes any substantive sense.  If the scale is arbitrary and not
well-known, and you are lucky enough that some transformation creates
fewer problems than it solves, then it may be sensible to use the BoxCox
transformation, even if the transformation is bizarre.   OTOH, if the
scale is known, or inherently sensible, then transformation is less
likely to be the ideal solution, and, if a transformation is necessary,
it should be one that preserves the sense of the variable.

So......a colleague wanted to log transform a ratio.  WAIT, I said.
Does it make any substantive sense to change a ratio into a difference?
(It did not).  OTOH, if called for, an inverse transformation of a ratio
can make sense (changing miles per gallon to gallons per mile)

As always, I think the substantive question should drive the statistics,
not vice versa


HTH

Peter

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On 2/27/09, nuria < XXXX@XXXXX.COM > wrote:
> What is the difference between:
>
> option 1
> proc genmod;
> class treat;
> model count = treat / dist=poisson link=log;
> run;
>
> option2
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>
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> run;
>
> Thanks!
>

8. POISSON: GENMOD VS. LOG TRANSFORMED DATE & MIXED