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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