Hi Chris lme (LMMs only) can handle unequal variances of residuals at the lowest level in your hierarchy using variance functions: this isn't really what you describe below. This is described in Pinhero and Bates, but for beginners, best to look at Zuur "Mixed Effects Models and Extensions in Ecology with R" which gives a good overview.
lmer can of course do LMMs and GLMMs, but so far as I know it cannot do unequal residual variances by groups. Both can do unequal random effect (i.e. the hierarchical levels above the lowest level residuals) variances by group, but with anything more than two groups this does get a bit complicated. You put the factor to the left of the | in the random part as if it were a random slope: for interpretation it can be easier to reformulate your factor as indicator variables of 1's and 0's in separate columns. Regards Mike -----Original Message----- From: Chris Howden [mailto:ch...@trickysolutions.com.au] Sent: 27 June 2011 06:01 To: Dunbar, Michael J.; Cliff Beall; r-sig-ecology@r-project.org Subject: RE: [R-sig-eco] Partially non-independent ANOVA? Hi Mike, I'm rather new to mixed models, but am on a few lists at the moment to try and get my head around them. Your comments on using GLMM's on data with unequal variances gave me a very pleasant surprise! And after thinking about it I realised it was a fairly obvious use of GLMM's. Can I just confirm your comments though? If I was testing some factor that I knew each group had unequal variances could I test it using something like the following? lmer(response~factor + (1|factor), data=data)? Chris Howden Founding Partner Tricky Solutions Tricky Solutions 4 Tricky Problems Evidence Based Strategic Development, IP Commercialisation and Innovation, Data Analysis, Modelling and Training (mobile) 0410 689 945 (fax / office) ch...@trickysolutions.com.au Disclaimer: The information in this email and any attachments to it are confidential and may contain legally privileged information. If you are not the named or intended recipient, please delete this communication and contact us immediately. Please note you are not authorised to copy, use or disclose this communication or any attachments without our consent. Although this email has been checked by anti-virus software, there is a risk that email messages may be corrupted or infected by viruses or other interferences. No responsibility is accepted for such interference. Unless expressly stated, the views of the writer are not those of the company. Tricky Solutions always does our best to provide accurate forecasts and analyses based on the data supplied, however it is possible that some important predictors were not included in the data sent to us. Information provided by us should not be solely relied upon when making decisions and clients should use their own judgement. -----Original Message----- From: r-sig-ecology-boun...@r-project.org [mailto:r-sig-ecology-boun...@r-project.org] On Behalf Of Dunbar, Michael J. Sent: Thursday, 23 June 2011 10:59 PM To: Cliff Beall; r-sig-ecology@r-project.org Subject: Re: [R-sig-eco] Partially non-independent ANOVA? Dear Cliff Using a mixed-effects model (also variously termed multilevel or hierarchical model) could well be what you are looking for. These are implemented in several R packages. In your study, in mixed model parlance, patient identifier would be a random effect (you don't say how many patients), with some patients having one measurement and some two, and treatment (control or periodontitis) would be a fixed effect. How or whether you code the sites (which would be another fixed effect) depends on whether they are of interest. These approaches are very powerful, but I wouldn't advise just dipping into them - it can be hard to get things set up correctly if you just want "a test", they repay careful study. On the bonus side, once you're familiar with them you start seeing the everything in a hierarchical mindset! You can model unequal variances by group, although you need to be careful in model specification. Regards Mike -----Original Message----- From: r-sig-ecology-boun...@r-project.org [mailto:r-sig-ecology-boun...@r-project.org] On Behalf Of Cliff Beall Sent: 22 June 2011 17:18 To: r-sig-ecology@r-project.org Subject: [R-sig-eco] Partially non-independent ANOVA? Dear group- I was wondering if someone could point me towards the correct statistical test and R-function for my situation. I am looking at oral bacteria and have three groups of samples, one from control subjects, and two that are derived from different sites in patients with periodontitis. So I have one totally independent group, and two that have samples that are paired. I would like to do a test just to say that there are differences between the three groups, something like an ANOVA, but that deals with the lack of independence of the two groups. By the way, for what I am measuring it looks like normality is a reasonable assumption but equal variances are probably not. Thanks, Cliff Beall Research Assistant Professor Division of Oral Biology College of Dentistry The Ohio State University beal...@osu.edu _______________________________________________ R-sig-ecology mailing list R-sig-ecology@r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-ecology -- This message (and any attachments) is for the recipient ...{{dropped:10}} _______________________________________________ R-sig-ecology mailing list R-sig-ecology@r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-ecology