Re: st: Moderation effect by splitting the sample

[Rebecca Pope <rebecca.a.pope@gmail.com>]

Thanks David. Sorry for the delay responding. I've been away for the holidays.

Your earlier post is missing from my inbox. All I have is the chain
with Maarten's response. I went back and read the archives for context
on your response to my query, but I appreciate you reposting the
paper.

I see the value of using the graphs presented for conveying
information to non-technical audiences. I'm still not convinced after
reading this paper that discretizing a continuous regressor is a good
idea when conducting inferential analysis. Reading your earlier posts,
the ones you cited, and the subtext of the paper, I am left with the
impression that this is the general consensus.

I'm going to rephrase my original question to my intent rather than
what it strictly said:
Is there any econometric (or statistical if you prefer) reason to
choose to conduct a "split" analysis unless you have natural groups
and a strong theoretical reason to not force equality in their
variances?

Thanks,
Rebecca



On Fri, Dec 21, 2012 at 11:09 AM, David Radwin <dradwin@mprinc.com> wrote:
> Rebecca,
>
> Sometimes you want to present a result in a simpler or less technical way,
> perhaps to a non-expert audience. It is often easier and more parsimonious
> to compare two groups, whether verbally or in a table or graph. The cost
> is some loss in power. But it may be possible to present the continuous
> relationship, too, perhaps in an appendix or some other less prominent
> fashion.
>
> For an example of how income (a continuous variable that could be split
> into two groups for simplicity) is related to voting in US presidential
> elections, please see the work I referred to earlier:
>
> Gelman, A., & Park, D. K. (2009). Splitting a predictor at the upper
> quarter or third and the lower quarter or third. The American
> Statistician, 63(1), 1-8.
> http://www.stat.columbia.edu/~gelman/research/published/thirds5.pdf
>
> David
> --
> David Radwin
> Senior Research Associate
> MPR Associates, Inc.
> 2150 Shattuck Ave., Suite 800
> Berkeley, CA 94704
> Phone: 510-849-4942
> Fax: 510-849-0794
>
> www.mprinc.com
>
>
>> -----Original Message-----
>> From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-
>> statalist@hsphsun2.harvard.edu] On Behalf Of Rebecca Pope
>> Sent: Thursday, December 20, 2012 1:24 PM
>> To: statalist@hsphsun2.harvard.edu
>> Subject: Re: st: Moderation effect by splitting the sample
>>
>> Maarten wrote: "Splitting a sample means that you added an interaction
>> term with all variables. This is typically not what you want, and
>> often leads to a severe loss of power."
>>
>> My understanding is that you would only do this when you have natural
>> groups and a strong theoretical reason to not force equality in their
>> variances. Is there any other situation where this approach is
>> warranted?
>>
>>
>> Thanks,
>> Rebecca
>>
>>
>>
>>
>> On Thu, Dec 20, 2012 at 1:53 PM, Maarten Buis <maartenlbuis@gmail.com>
>> wrote:
>> > On Thu, Dec 20, 2012 at 8:42 PM, Ebru Ozturk wrote:
>> >> For non-linear models, I want to test the moderation effect of X
>> variable. Can I test this moderation effect by spliting the sample
>> according to X variable (moderator)?
>> >
>> > That is typically inefficient. Moderation is just an interaction
>> > effect. Splitting a sample means that you added an interaction term
>> > with all variables. This is typically not what you want, and often
>> > leads to a severe loss of power. It is even worse if your variable x
>> > is continuous and you are splitting the sample by first making it
>> > categorical by splitting it at some arbitrary number (e.g. the median
>> > from your previous question). That is a very bad idea, as you would
>> > loose even more information that way. Instead you should just add your
>> > interaction effect and interpret it correctly. Various examples are
>> > given here:
> <http://www.maartenbuis.nl/publications/interactions.html>.
>> >
>> > -- Maarten
>> >
>> > ---------------------------------
>> > Maarten L. Buis
>> > WZB
>> > Reichpietschufer 50
>> > 10785 Berlin
>> > Germany
>> >
>> > http://www.maartenbuis.nl
>> > ---------------------------------
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