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]

From |
"David Radwin" <dradwin@mprinc.com> |

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

Subject |
RE: st: Moderation effect by splitting the sample |

Date |
Wed, 2 Jan 2013 08:53:23 -0800 (PST) |

Rebecca, Aside from the simplicity argument outlined below, I can't think of a reason. 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: Wednesday, January 02, 2013 8:33 AM > To: statalist@hsphsun2.harvard.edu > Subject: Re: st: Moderation effect by splitting the sample > > 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 > >> > --------------------------------- * * 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: Moderation effect by splitting the sample***From:*Rebecca Pope <rebecca.a.pope@gmail.com>

**References**:**Re: st: Moderation effect by splitting the sample***From:*Rebecca Pope <rebecca.a.pope@gmail.com>

- Prev by Date:
**RE: st: Stata 13 wishlist** - Next by Date:
**Re: st: Moderation effect by splitting the sample** - Previous by thread:
**Re: st: Moderation effect by splitting the sample** - Next by thread:
**Re: st: Moderation effect by splitting the sample** - Index(es):