Re: [R] Interaction term in lmer

From: Martin Henry H. Stevens <HStevens_at_muohio.edu>
Date: Fri, 01 Jun 2007 14:25:41 -0400

HI Emine,
The parentheses specify a variable as random or fixed. You are using center for random intercepts, but fixed for interactions. You may want
> mer(cbind( yvect, nvect-yvect) ~ trt + ( 1 | center) + (1 |
> trt:center),
> family = binomial, niter = 25, method = "Laplace", control = list
> (usePQL =
> FALSE) )
Cheers,
Hank

On Jun 1, 2007, at 2:01 PM, emine özgür Bayman wrote:

> Dear R users,
> I'm pretty new on using lmer package. My response is binary and I
> have fixed
> treatment effect (2 treatments) and random center effect (7
> centers). I want
> to test the effect of treatment by fitting 2 models:
>
> Model 1: center effect (random) only
> Model 2: trt (fixed) + center (random) + trt*center interaction.
>
> Then, I want to compare these 2 models with Likelihood Ratio Test.
> Here are
> my lmer codes that I don't feel comfortable about their correctness.
>
> model1 <- try(lmer(cbind( yvect, nvect-yvect) ~ 1 + (1 | center),
> family = binomial, niter = 25, method = "Laplace", control = list
> (usePQL =
> FALSE) ))
>
> model2 <- try(lmer(cbind( yvect, nvect-yvect) ~ trt*center + ( 1 |
> center)
> ,
> family = binomial, niter = 25, method = "Laplace", control = list
> (usePQL =
> FALSE) ))
>
>
> (I have attached outputs below)
> What I don't understand is; I thought in model2 I have defined
> center effect
> as a random effect. Then how come I got center effects and trt*center
> interactions under the fixed effects list on the output? Probably I
> didn't
> understand how to set up these models in lmer. Could anyone help me
> about
> this?
>
> Thanks a lot for your help...
>
> Emine
>
> model1 <- try(lmer(cbind( yvect, nvect-yvect) ~ 1 + (1 | center),
> family = binomial, niter = 25, method = "Laplace", control = list
> (usePQL =
> FALSE) ))
>
>> summary(model1)
> Generalized linear mixed model fit using Laplace
> Formula: cbind(yvect, nvect - yvect) ~ 1 + (1 | center)
> Family: binomial(logit link)
> AIC BIC logLik deviance
> 236.817 238.0951 -116.4085 232.817
> Random effects:
> Groups Name Variance Std.Dev.
> center (Intercept) 0.088127 0.29686
> number of obs: 14, groups: center, 7
>
> Estimated scale (compare to 1) 0.2672612
>
> Fixed effects:
> Estimate Std. Error z value Pr(>|z|)
> (Intercept) -0.32084 0.14709 -2.1812 0.02917 *
> ---
> Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
>
>
> ##################
> model2 <- try(lmer(cbind( yvect, nvect-yvect) ~ trt*center + ( 1 |
> center)
> ,
> family = binomial, niter = 25, method = "Laplace", control = list
> (usePQL =
> FALSE) ))
>
>> summary(model2)
> Generalized linear mixed model fit using Laplace
> Formula: cbind(yvect, nvect - yvect) ~ trt * center + (1 | center)
> Family: binomial(logit link)
> AIC BIC logLik deviance
> 30 39.58586 -1.547024e-07 3.094048e-07
> Random effects:
> Groups Name Variance Std.Dev.
> center (Intercept) 5e-10 2.2361e-05
> number of obs: 14, groups: center, 7
>
> Estimated scale (compare to 1) 0.2672612
>
> Fixed effects:
> Estimate Std. Error z value Pr(>|z|)
> (Intercept) -1.060869 0.065372 -16.2282 < 2e-16 ***
> trt2 1.118029 0.086842 12.8743 < 2e-16 ***
> center2 -0.325428 0.504256 -0.6454 0.51869
> center3 -0.325440 0.504258 -0.6454 0.51868
> center4 0.655407 0.413449 1.5852 0.11292
> center5 -0.325433 0.504256 -0.6454 0.51869
> center6 -0.325421 0.504255 -0.6454 0.51870
> center7 0.655422 0.413448 1.5853 0.11291
> trt2:center2 0.673737 0.651313 1.0344 0.30094
> trt2:center3 -0.137183 0.651314 -0.2106 0.83318
> trt2:center4 -0.307083 0.583845 -0.5260 0.59891
> trt2:center5 -0.137203 0.651314 -0.2107 0.83316
> trt2:center6 1.654555 0.712419 2.3224 0.02021 *
> trt2:center7 0.673705 0.651311 1.0344 0.30096
> ---
>
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Dr. Hank Stevens, Assistant Professor
338 Pearson Hall
Botany Department
Miami University
Oxford, OH 45056

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http://www.cas.muohio.edu/~stevenmh/
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http://www.muohio.edu/botany/

"E Pluribus Unum"



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https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. Received on Fri 01 Jun 2007 - 18:33:32 GMT

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