Dear Doug, Your point is correct, of course, but if people are interested in computing marginal means (or marginal cell means), then they can do so simply and don't need a statistical model. I think that when such a model is fit, interest is typically in conditioning on the other explanatory variables.
(Also see my responses to Hadley and Frank's points.) Regards, John ------------------------------ John Fox, Professor Department of Sociology McMaster University Hamilton, Ontario, Canada web: socserv.mcmaster.ca/jfox > -----Original Message----- > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On > Behalf Of Douglas Bates > Sent: June-08-08 1:58 PM > To: John Fox > Cc: Dieter Menne; [EMAIL PROTECTED] > Subject: Re: [R] lsmeans > > 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. > > If there are no interactions with the "other terms" and if the values > chosen for those other terms based on the observed data are indeed > typical of the values for which we wish to make inferences with the > model then these conditional cell means may tell us something about > the marginal cell means. But if either of those conditions fails then > these conditional means can be very different from the marginal means. > > I wouldn't have any problem at all with providing conditional cell > means, especially if the user were required to specify the values at > which to fix the other terms in the model, but that is not what people > think they are getting. I don't want to encourage them in their > delusions by letting them think i can evaluate marginal cell means as > a single, conditional evaluation. > > > > -----Original Message----- > > > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] > > On > > > Behalf Of Dieter Menne > > > Sent: June-07-08 4:36 AM > > > To: [EMAIL PROTECTED] > > > Subject: Re: [R] lsmeans > > > > > > John Fox <jfox <at> mcmaster.ca> writes: > > > > > > > I intend at some point to extend the effects package to linear and > > > > generalized linear mixed-effects models, probably using lmer() rather > > > > than lme(), but as you discovered, it doesn't handle these models now. > > > > > > > > It wouldn't be hard, however, to do the computations yourself, using > > > > the coefficient vector for the fixed effects and a suitably > constructed > > > > model-matrix to compute the effects; you could also get standard > errors > > > > by using the covariance matrix for the fixed effects. > > > > > > > > > > >> Douglas Bates: > > > https://stat.ethz.ch/pipermail/r-sig-mixed-models/2007q2/000222.html > > > >> > > > My big problem with lsmeans is > > > that I have never been able to understand how they should be > > > calculated and, more importantly, why one should want to calculate > > > them. In other words, what do lsmeans represent and why should I be > > > interested in these particular values? > > > >> > > > > > > Truly Confused, torn apart by the Masters > > > > > > Dieter > > > > > > ______________________________________________ > > > 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. > > > > ______________________________________________ > > 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. > > > > ______________________________________________ > 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. ______________________________________________ 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.
