[R] question about lmer--different answers from different versions of R?

From: Feldman, Tracy <tsfeldman_at_noble.org>
Date: Tue 28 Feb 2006 - 01:52:47 EST


To whom it may concern:  

I am using lmer for a statistical model that includes non-normally distributed data and random effects. I used this same function in the most recent version of R as of fall 2005, and have re-done some of the same analyses using all of the same files, but with the newest version of R (2.2.1). I get answers that are not exactly the same (although I do get the same general patterns of significance). Is there any reason why an updated version of lmer in R would give different answers than the same function in a previous version? See example below.  

I appreciate your help. Please direct answers to tsfeldman@noble.org  

Sincerely,  

Tracy S. Feldman  

Postdoc, The Samuel Roberts Noble Foundation    

Example:  

The model formulas (same in both):  

fitvisit1.1<-lmer(visitsR~dist + ctrt + (1|rep) + (1|field), family = poisson, method = "ML")

fitvisit1.3<-lmer(visitsR~dist + (1|rep) + (1|field), family = poisson, method = "ML")  

Done in fall 2005 (I forget the most recent version then):  

> anova(fitvisit1.1, fitvisit1.3, test = "Chisq")

Data:

Models:

fitvisit1.3: visitsR ~ dist + (1 | rep) + (1 | field)

fitvisit1.1: visitsR ~ dist + ctrt + (1 | rep) + (1 | field)

            Df AIC BIC logLik Chisq Chi Df Pr(>Chisq)

fitvisit1.3 7 855.11 878.74 -420.56

fitvisit1.1 8 856.63 883.63 -420.31 0.4856 1 0.4859    

Done in Feb 2006 (in R 2.2.1):  

> anova(fitvisit1.1, fitvisit1.3, test = "Chisq")

Data:

Models:

fitvisit1.3: visitsR ~ dist + (1 | rep) + (1 | field)

fitvisit1.1: visitsR ~ dist + ctrt + (1 | rep) + (1 | field)

            Df AIC BIC logLik Chisq Chi Df Pr(>Chisq)

fitvisit1.3 6 875.04 895.29 -431.52

fitvisit1.1 7 876.69 900.32 -431.35 0.3487 1 0.5549    

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