I have not seen an SPSS source on this point but there are a few discussions about it by SAS users. One source is the following: http://support.sas.com/onlinedoc/913/getDoc/en/statug.hlp/glm_sect34.htm
What SAS calls "LS means" or "Least Squares means" is equivalent to SPSS' estimated marginal means (emm). Ordinary means are referred to as "arithmetic means"; see the following for the distinction: http://onbiostatistics.blogspot.com/2009/04/least-squares-means-marginal-means-vs.html If you have a "balanced" design (e.g., a factorial design with constant N for each cell), then the LS/emm means will be the same as the arithmetic means. In an unbalanced design, the type of sums of square calculation you use can produce differences between the two (e.g., Type I SS or sequential SS vs.Type III SS or unweighted means). See the following for a worked example: http://www.public.iastate.edu/~dnett/S402/wlsmeans.pdf -Mike Palij New York University [email protected] ------------------- Original Message ------------------------ On Wed, 14 Dec 2011 12:09:11 -0800, Jim Clark wrote: Hi I have not observed this problem and, as Stuart observed, generally see EMMs that differ from originals when covariates are involved. One thing to check might be whether it occurs with unequal ns per condition / cell? Could SPSS be estimating effects controlling for any confounding between factors? If you have an example of this issue, I would be happy to see it. Take care Jim >>> Stuart McKelvie <[email protected]> 14-Dec-11 12:38 PM >>> Dear Tipsters, I have puzzled over this question for a long time. When conducting an ANOPVA in SPSS, we have two options for obtaining means in each condition. 1. Click on descriptives. This gives means, standard deviations and sample sizes for all main effects and interactions. They appear at the beginning of the printout. 2. Under options, you can specify which means you want. This gives means and standard errors for the effects and interactions that you specify. They appear at the end of the printout. My question is: Why are these means sometimes different from the ones in the descriptives in 1? I think I have seen the term "estimated marginal means", with a reference to covariates, but if there are no covariates (it is ANOVA not ANCOVA), I do not understand this. I would have thought that a mean was a mean in ANOVA. --- You are currently subscribed to tips as: [email protected]. To unsubscribe click here: http://fsulist.frostburg.edu/u?id=13090.68da6e6e5325aa33287ff385b70df5d5&n=T&l=tips&o=14893 or send a blank email to leave-14893-13090.68da6e6e5325aa33287ff385b70df...@fsulist.frostburg.edu
