[R] lmer specification for random effects: contradictory reults

Mon Nov 25 14:48:17 CET 2013

Dear Benedetta,

I think you might want (1+T+Z|subject) as random effects  rather than (1+T|subject) + (1 + Z|subject). The latter has two random intercepts per subject: a recipe for disaster.

Follow-up posts should only go to the mixed models mailing list which I'm cc'ing.

Best regards,

ir. Thierry Onkelinx
Instituut voor natuur- en bosonderzoek / Research Institute for Nature and Forest
team Biometrie & Kwaliteitszorg / team Biometrics & Quality Assurance
Kliniekstraat 25
1070 Anderlecht
Belgium
+ 32 2 525 02 51
+ 32 54 43 61 85
Thierry.Onkelinx op inbo.be
www.inbo.be

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~ Sir Ronald Aylmer Fisher

The plural of anecdote is not data.
~ Roger Brinner

The combination of some data and an aching desire for an answer does not ensure that a reasonable answer can be extracted from a given body of data.
~ John Tukey

-----Oorspronkelijk bericht-----
Van: r-help-bounces op r-project.org [mailto:r-help-bounces op r-project.org] Namens Benedetta Cesqui
Verzonden: maandag 25 november 2013 11:13
Aan: r-help op r-project.org
Onderwerp: [R] lmer specification for random effects: contradictory reults

Hi All,

I was wondering if someone could help me to solve this issue with lmer.
In order to understand the best mixed effects model to fit my data, I compared the following options according to the procedures specified in many papers (i.e. Baayen <http://www.google.it/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CDsQFjAA
&url=http%3A%2F%2Fwww.ualberta.ca%2F~baayen%2Fpublications%2FbaayenDavidsonB
ates.pdf&ei=FhqTUoXuJKKV7Abds4GYBA&usg=AFQjCNFst7GT7mBX7w9lXItJTtELJSKWJg&si
g2=KGA5MHxOvEGwDxf-Gcqi6g&bvm>  R.H. et al 2008) Here, dT_purs is the response variable, T and Z are the fixed effects, and subject is the random effect. Random and fixed effects are crossed.:

mod0 <- lmer(dT_purs ~ T + Z + (1|subject), data = x)
mod1 <- lmer(dT_purs ~ T + Z + (1 +tempo| subject), data = x)
mod2 <- lmer(dT_purs ~ T + Z + (1 +tempo| subject) + (1+ Z| subject), data =
x)
mod3 <- lmer(dT_purs ~ T * Z + (1 +tempo| subject) + (1+ Z| subject), data =
x)
mod4 <- lmer(dT_purs ~ T * Z + (1| subject), data = x)

anova(mod0, mod1,mod2, mod3, mod4)

Data: x

Models:

mod0: dT_purs ~ T + Z + (1 | subject)

mod4: dT_purs ~ T * Z + (1 | subject )

mod1: dT_purs ~ T + Z + (1 + T| subject)

mod2: dT_purs ~ T + Z + (1 + T| subject ) + (1 + Z | subject)

mod3: dT_purs ~ T * Z + (1 + T| subject) + (1 + Z | subject)

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

mod0  5 -689.81 -669.46 349.91  -699.81

mod4  6 -689.57 -665.14 350.78  -701.57  1.7532      1   0.185473

mod1  7 -689.12 -660.62 351.56  -703.12  1.5504      1   0.213070

mod2 10 -695.67 -654.97 357.84  -715.67 12.5563      3   0.005701 **

mod3 11 -695.83 -651.05 358.92  -717.83  2.1580      1   0.141825

---

Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1

It turns out that mod2 has the right level of complexity for this dataset.

However when I looked at its summary, I got a correlation of -0.87 for the
random effects relative to the T effect and -1 for the random effects
relatively to the Z.

summary(mod2)

Linear mixed model fit by maximum likelihood ['lmerMod']

Formula: dT_purs ~T + Z + (1 + T | subject) + (1 + Z | subject)

   Data: x

      AIC       BIC    logLik  deviance

-695.6729 -654.9655  357.8364 -715.6729

Random effects:

Groups     Name        Variance  Std.Dev. Corr

 subject   (Intercept) 0.0032063 0.05662

            T       0.0117204 0.10826  -0.87

subject.1 (Intercept) 0.0005673 0.02382

            Z           0.0025859 0.05085  1.00

 Residual               0.0104551 0.10225

Number of obs: 433, groups: soggetto, 7

Fixed effects:

            Estimate Std. Error t value

(Intercept)  0.02489    0.03833   0.650

T        0.52010    0.05905   8.808

Z           -0.09019    0.02199  -4.101

Correlation of Fixed Effects:

      (Intr) tempo

T -0.901

Z      0.218 -0.026

If I understand correctly what the correlation parameters reported in the
table are, the correlation of 1 means that, for the Z effects the random
intercept is perfectly collinear with the random slope. Thus, we fit the
wrong model. A random intercept only model would have sufficed.

Am I correct?

If so, should I take mod1 (mod1 <- dT_purs ~ T + Z + (1 + T | subject )
instead of mod2 to fit my data?

Why are these results contradictory?

Finally is a correlation value of -0.87 a too high or an acceptable value ?

Thanks for help me in advance!

Best

Benedetta

---

Benedetta Cesqui, Ph.D.

Laboratory of Neuromotor Physiology

IRCCS Fondazione Santa Lucia

Via Ardeatina 306

00179 Rome, Italy

tel: (+39) 06-51501485

fax:(+39) 06-51501482

E_mail:  b.cesqui op hsantalucia.it

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