[R-sig-ME] Correlation of -1.00 in a mixed model

[Manuel Spínola]

Dear list members,

A few days ago somebody posted a problem like the want that I have now, 
I got a correlation value of - 1.00.  I would like to have some help in 
the context of my model fitting.
My model is a random intercept random slope model (repeated measure 
design).  Variable "time" has been centered.
What will be an alternative model?

 > modelo7 = lmer(ipa ~ time + (time | localidad), data=ipa)
 > summary(modelo7)
Linear mixed model fit by REML
Formula: ipa ~ time + (time | localidad)
   Data: ipa
  AIC  BIC logLik deviance REMLdev
 1917 1937 -952.4     1913    1905
Random effects:
 Groups      Name           Variance     Std.Dev.    Corr  
 localidad  (Intercept)        2232.31    47.247
                   time                545.97     23.366   -1.000
 Residual                           450.92     21.235
Number of obs: 196, groups: localidad, 49

Fixed effects:
            Estimate Std. Error t value
(Intercept)   42.852      6.918   6.194
time         -19.746      3.603  -5.480

Correlation of Fixed Effects:
     (Intr)
time -0.904

Thank you very much in advance.
Best,

Manuel

-- 
Manuel Spínola, Ph.D.
Instituto Internacional en Conservación y Manejo de Vida Silvestre
Universidad Nacional
Apartado 1350-3000
Heredia
COSTA RICA
mspinola at una.ac.cr
mspinola10 at gmail.com
Teléfono: (506) 2277-3598
Fax: (506) 2237-7036