Thanks Rolf! Very helpful. Maybe my question was not as stupid as I had
feared!


On Thu, Feb 20, 2014 at 4:39 PM, Rolf Turner <r.turner@auckland.ac.nz>wrote:

>
> In general anova() is *not* really quite smart enough. Consider the case
> of ordinary garden-variety linear models:
>
> x <- seq(0,1,length=20)
> set.seed(42)
> y <- rnorm(20)
> f1 <- lm(y ~ x)
> f2 <- lm(y ~ x + I(x^2))
> anova(f2,f1)
>
> This gives a negative degrees of freedom and a negative sum of squares.
>  The F statistic and p-value are correct however, so the "not quite smart
> enough" aspect is really just cosmetic.  Still, those negative values are
> disconcerting.
>
> It does appear that the anova() function used on "mer" objects is smarter
> than anova.lm().  It is hard/impossible to figure out just *what* is being
> used when one calls anova(gm2,gm1) because of these bloody awful
> obscurantist S4 classes and methods which seem to be involved here.
>
> Bottom line:  In general the appropriate syntax for anova() would appear
> to be anova(reduced model, full model) as the OP stated.  But for "mer"
> objects it doesn't matter.
>
> cheers,
>
> Rolf
>
>
> On 21/02/14 09:53, Steve Taylor wrote:
>
>> Both ways work.  I think anova() is smart enough to figure out which
>> model is the smaller one.
>>
>> library(lme4)
>> example(glmer)
>> # Compare these outputs...
>> anova(gm1,gm2)
>> anova(gm2,gm1)
>>
>> cheers,
>>       Steve
>> ____________________________________________
>>
>> Steve Taylor
>>     Biostatistician
>>     Faculty of Health and Environmental Sciences
>>     AUT University
>>     http://www.aut.ac.nz/profiles/steve-taylor
>> ____________________________________________
>>
>> -----Original Message-----
>> From: r-sig-mixed-models-bounces@r-project.org [mailto:
>> r-sig-mixed-models-bounces@r-project.org] On Behalf Of Juliet Hannah
>> Sent: Friday, 21 February 2014 5:22a
>> To: r-sig-mixed-models@r-project.org
>> Subject: [R-sig-ME] order of models when using anova
>>
>> All,
>>
>> When using the anova command to compare nested models I thought the order
>> was
>>
>> anova(reduced model, full model)
>>
>> Where the 'full model' has more terms.
>>
>>
>> However, looking at the slides on:
>>
>> http://lme4.r-forge.r-project.org/slides/2009-07-21-
>> Seewiesen/6Interactions-4a4.pdf
>>
>>
>> pg 3 (comparing the model fits), it looks like I had this backward.
>>
>> Did I have it backward, or is lme4 different, or is the order not
>> important?
>>
>> Thanks,
>>
>> Juliet
>>
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>>
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>>
>

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