I know that I have heard and seen this before, but I am not completely clear on why and unsure what a general rule may be for the number of predictors you can have.

http://support.sas.com/documentatio...TML/default/viewer.htm#statug_glm_sect054.htm

Here is a link to a linear mixed model that had 44 observations and 2 categorical predictors (e.g., one had three levels and the other had 6 levels) as well as their interaction term. If we think about this from a standard linear model you would have 7 terms via dummy coding plus the interaction term(s), which is around 5 observations per term?

Thanks in advance for some input!