Thank you for your answer Ben B., it is helpful. The post on Fixed vs Random effects is particularly interesting.
I had in mind to create a fixed interaction, as you propose (time*A). I actually wanted to compare it with a random interaction, so to decide on which model to go with based on this comparison... I would just ask one more thing, if someone is feeling like answering... what's the difference between (1 | A:B) and (1 | A/B) ? I though it was exactly the same thing, but the (1 | A/B) form was used in older versions of the lmer package...? Regards ________________________________ De : Ben Bolker <bbol...@gmail.com> À : r-h...@stat.math.ethz.ch Envoyé le : Mardi 13 août 2013 15h55 Objet : Re: [R] Lme4 and syntax of random factors Robert U <tacsunday <at> yahoo.fr> writes: > > Dear > R-users, > [snip] This question probably belongs on r-sig-mixed-mod...@r-project.org . Followups there, please. > Let's say that I have 2 random effects, A (e.g. species, k=2) and B (e.g. individuals, n=100). I made some research about model syntax, and I have the understanding that everything at the left side of the random parameter is about SLOPE and everything at the right side about intercept : You really can't practically fit a random effect to 2 species (see http://glmm.wikidot.com/faq#fixed_vs_random > + (1 |B) > would give me an intercept per individual. > + (1 |A) > would give me an intercept per species. yes > + (1 |A:B) > would give me an intercept per individuals with nested effect (individual > inside species) This would be the same as (1|B) if the individuals are uniquely identified. Otherwise you probably want (1|A/B) [except that you can't really fit a random effect for k=2, as discussed above] > I would like to have random slopes per species. So I thought I > could do something like that : Probably not feasible. > + (A |B) so to have an intercept per individual and a slope value > per species. Graphically, I would therefore obtain 100 lines with > 100 different intercepts and 2 possible slopes (1 per > species). However, when I extract random parameter values (ranef()), > I have : what variable is your slope with respect to? Suppose it's time. Then I would recommend ~ A*time + (1|A:B) which will fit a (FIXED effect) interaction between species and time (different slopes and intercepts for each species), and a random intercept per individual. ______________________________________________ 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. [[alternative HTML version deleted]]
______________________________________________ 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.
