Dear Hadley, Unfortunately, the term "marginal" gets used in two quite different ways, and Searle's "population marginal means" would, I believe, be more clearly called "population conditional means" or "population partial means." This is more or less alternative terminology for "least-squares means" (to which Searle rightly objects).
Regards, John ------------------------------ John Fox, Professor Department of Sociology McMaster University Hamilton, Ontario, Canada web: socserv.mcmaster.ca/jfox > -----Original Message----- > From: hadley wickham [mailto:[EMAIL PROTECTED] > Sent: June-08-08 2:52 PM > To: Douglas Bates > Cc: John Fox; Dieter Menne; [EMAIL PROTECTED] > Subject: Re: [R] lsmeans > > On Sun, Jun 8, 2008 at 12:58 PM, Douglas Bates <[EMAIL PROTECTED]> wrote: > > On 6/7/08, John Fox <[EMAIL PROTECTED]> wrote: > >> Dear Dieter, > >> > >> I don't know whether I qualify as a "master," but here's my brief take on > >> the subject: First, I dislike the term "least-squares means," which seems > to > >> me like nonsense. Second, what I prefer to call "effect displays" are > just > >> judiciously chosen regions of the response surface of a model, meant to > >> clarify effects in complex models. For example, a two-way interaction is > >> displayed by absorbing the constant and main-effect terms in the > interaction > >> (more generally, absorbing terms marginal to a particular term) and > setting > >> other terms to typical values. A table or graph of the resulting fitted > >> values is, I would argue, easier to grasp than the coefficients, the > >> interpretation of which can entail complicated mental arithmetic. > > > > I like that explanation, John. > > > > As I'm sure you are aware, the key phrase in what you wrote is > > "setting other terms to typical values". That is, these are > > conditional cell means, yet they are almost universally misunderstood > > - even by statisticians who should know better - to be marginal cell > > means. A more subtle aspect of that phrase is the interpretation of > > "typical". The user is not required to specify these typical values - > > they are calculated from the observed data. > > > > How does Searle's "population marginal means" fit in to this? The > paper describes a PMM as "expected value of an observed marginal mean > as if there were one observation in every cell." - which was what I > thought happened in the effects display. Is this a subtly on the > definition of typical, or is that PMM's are only described for pure > ANOVA's (i.e. no continuous variables in model)? > > Hadley > > -- > http://had.co.nz/ ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
