non-positive definite matrix
Hi Tim-- All methods in the lme4 package (i.e., lmer, glmer, nlmer) use iterative optimization routines. You can see the iterations if you set "verbose = TRUE". It is true that linear mixed models are easier to fit, but it is still an iterative fitting procedure. I have received the error you mention several times, which always (I believe) stemmed from one of two sources: 1. There was redundancy in the fixed-effects. 2. I was trying to fit a very complicated model to not so much data. As a proposed "solution" for either 1 or 2 above, I'd suggest starting with a very simple model and work up slowly. Ideally, you could diagnose quickly where the error occurs -- that is, is there some predictor or random-effect that seems to trigger the error when it enters the model. If those don't seem helpful or not relevant, then more info about your data, your model, and ideally a reproducible example will get you a lot more helpful responses. [Hope your staying cool in Thousand Oaks...] cheers, Dave
Tim wrote:
Hello, I'm trying to use lmer to fit a linear mixed effects model to some data. Unfortunately, lmer fails, saying "Error in mer_finalize(ans) : Downdated X'X is not positive definite." While this may be a problem with my setup, I've looked over it several times, so I think this is more likely a result of my data. A quick search of the internet suggests that sometimes, the random errors in real data are such that the resulting matrices are mathematically unacceptable. My one thought is that I might be able to avoid this problem by using a function which fits a model via iteration/optimization. Based on a very rough understanding of lmer, from Bates's book, my impression is that *linear* mixed models are fit via some matrix method (akin to vanilla least-squares regression), while generalized mixed models are fit via optimization (similar to glm). If this is true, then if I could get glmer to fit my lmm via optimization, then perhaps I could fit this model to my data without needing to tweak the data. I would greatly appreciate any thoughts or advice any of you might have on this problem. Thanks, Tim Handley Fire Effects Monitor Santa Monica Mountains National Recreation Area 401 W. Hillcrest Dr. Thousand Oaks, CA 91360 805-370-2300 x2412
Dave Atkins, PhD Research Associate Professor Department of Psychiatry and Behavioral Science University of Washington datkins at u.washington.edu Center for the Study of Health and Risk Behaviors (CSHRB) 1100 NE 45th Street, Suite 300 Seattle, WA 98105 206-616-3879 http://depts.washington.edu/cshrb/ (Mon-Wed) Center for Healthcare Improvement, for Addictions, Mental Illness, Medically Vulnerable Populations (CHAMMP) 325 9th Avenue, 2HH-15 Box 359911 Seattle, WA 98104 http://www.chammp.org (Thurs)