# Re: [R] How to assess significance of random effect in lme4

From: Spencer Graves <spencer.graves_at_pdf.com>
Date: Thu 18 Aug 2005 - 12:08:12 EST

Is there some reason you are NOT using "anova", as in "Examples" section of "?lmer"?

Permit me to summarize what I know about this, and I'll be pleased if someone else who thinks they know different would kindly enlighten me and others who might otherwise be misled if anything I say is inconsistent with the best literature available at the moment:

1. Doug Bates in his PhD dissertation and later in his book with Don Watts (1988) Nonlinear Regression Analysis and Its Applications (Wiley) split approximation errors in nonlinear least squares into "intrinsic curvature" and "parameter effects curvature". He quantified these two problems in the context of roughly three dozen published examples, if my memory is correct, and found that in not quite all cases, the parameter effects were at least an order of magnitude greater than the intrinsic curvature.
2. In nonnormal situations, maximum likelihood is subject to more approximation error -- intrinsic curvature -- than "simple" nonlinear least squares. However, I would expect this comparison to still be fairly accurate, even if the differences may not be quite as stark.
3. The traditional use of "standard errors" to judge statistical significance is subject to both intrinsic and parameter effects errors, while likelihood ratio procedures such as anova are subject only to the intrinsic curvature (assuming there are no substantive problems with nonconvergence). Consequently, to judge statistical significance of an effect, anova is usually substantially better than the so-called Wald procedure using approximate standard errors, and is almost never worse. If anyone knows of a case where this is NOT true, I'd like to know.
4. With parameters at a boundary as with variance components, the best procedure seems to double the p-value from a nested anova (unless the reported p-value is already large). This is because the 2*log(likelihood ratio) in such cases is roughly a 50-50 mixture of 0 and chi-square(1) [if testing only 1 variance component parameter]. This is supported by a substantial amount of research, including simulations discussed in a chapter in Pinheiro and Bates (2000) Mixed-Effects Models in S and S-Plus (Springer). The may be more accurate procedures available in the literature, but none so simple as this as far as I know.
```	  Comments?
spencer graves
```

p.s. It looks like fm@bVars is a list containing vectors of length 29 and 6 in your example. I don't know what they are, but I don't see how they can be standard errors in the usual sense.

Doran, Harold wrote:

> These are the posterior variances of the random effects (I think more
> properly termed "empirical" posteriors). Your model apparently includes
> three levels of random variation (commu, bcohort, residual). The first
> are the variances associated with your commu random effect and the
> second are the variances associated with the bcohort random effect.
>
> Accessing either one would require
>
> fm@bVar\$commu or fm@bVar\$bcohort
>
> Obviously, replace "fm" with the name of your fitted model.
>
> -----Original Message-----
> From: r-help-bounces@stat.math.ethz.ch
> [mailto:r-help-bounces@stat.math.ethz.ch] On Behalf Of Shige Song
> Sent: Wednesday, August 17, 2005 7:50 AM
> To: r-help@stat.math.ethz.ch
> Subject: Re: [R] How to assess significance of random effect in lme4
>
> Hi Harold,
>
> Thanks for the reply. I looked at my outputs using str() as you
> suggested, here is the part you mentioned:
>
> ..@ bVar :List of 2
> .. ..\$ commu : num [1, 1, 1:29] 5e-10 5e-10 5e-10 5e-10 5e-10 ...
> .. ..\$ bcohort: num [1, 1, 1:6] 1.05e-05 7.45e-06 6.53e-06 8.25e-06
> 7.11e-06 ...
>
> where commu and bcohort are the two second-level units. Are these
> standard errors? Why the second vector contains a series of different
> numbers?
>
> Thanks!
>
> Shige
>
> On 8/17/05, Doran, Harold <HDoran@air.org> wrote:
>

```>>
>>
>>You can extract the posterior variance of the random effect from the
>>bVar slot of the fitted lmer model. It is not a hidden option, but a
>>part of the fitted model. It just doesn't show up when you use
```

>
> summary().
>
```>>
>> Look at the structure of your object to see what is available using
```

>
> str().
>
```>>
>> However, your comment below seems to imply that it is incorrect for
>>lmer to report SDs instead of the standard error, which is not true.
>>That is a quantity of direct interest.
>>
>> Other multilevel programs report the same exact statistics (for the
>>most part). For instance, HLM reports the variances as well. If you
>>want the posterior variance of an HLM model you need to extract it.
>>
>>
>>
>> -----Original Message-----
>> From:   r-help-bounces@stat.math.ethz.ch on behalf of
>>Shige Song
>> Sent:   Wed 8/17/2005 6:30 AM
>> To:     r-help@stat.math.ethz.ch
>> Cc:
>> Subject:        [R] How to assess significance of random effect in
```

>
> lme4
>
```>>
>> Dear All,
>>
>> With kind help from several friends on the list, I am getting close.
>> Now here are something interesting I just realized: for random
>>effects, lmer reports standard deviation instead of standard error! Is
```

>
>
```>>there a hidden option that tells lmer to report standard error of
>>random effects, like most other multilevel or mixed modeling software,
```

>
>
```>>so that we can say something like "randome effect for xxx is
>>significant, while randome effect for xxx is not significant"? Thanks!
>>
>> Best,
>> Shige
>>
>> ______________________________________________
>> R-help@stat.math.ethz.ch mailing list
>>https://stat.ethz.ch/mailman/listinfo/r-help
>>http://www.R-project.org/posting-guide.html
>>
>>
>>
>>
>>
```

>
>
> ______________________________________________
> R-help@stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> http://www.R-project.org/posting-guide.html
>
> ______________________________________________
> R-help@stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
```--
Spencer Graves, PhD
Senior Development Engineer
PDF Solutions, Inc.
333 West San Carlos Street Suite 700
San Jose, CA 95110, USA

spencer.graves@pdf.com
www.pdf.com <http://www.pdf.com>
Tel:  408-938-4420
Fax: 408-280-7915

______________________________________________
R-help@stat.math.ethz.ch mailing list
https://stat.ethz.ch/mailman/listinfo/r-help