[R-sig-ME] (stupid) results interpretation question

[Ken Beath]

On 09/06/2009, at 9:29 PM, CL Pressland wrote:

> I have a seemingly stupid but fundamental question I need answering.
>
> I've had some strange plots and am now seriously questioning my  
> interpretation of the lmer output: if I have a fixed effects results  
> table that looks like this, can someone please check I'm  
> interpreting it correctly?
>
> ---------
> Fixed effects:
>           Estimate Std. Error t value
> 1	(Intercept) -1.58481    0.18585   -8.53
> 2	GM1          0.02400    0.20472    0.12
> 3	GM2          0.17941    0.15916    1.13
> 4	Morph1      -1.54068    0.02202  -69.97
> 5	GM1:Morph1  -0.21681    0.04701   -4.61
> 6	GM2:Morph1  -0.12910    0.03321   -3.89
> ---------
>
> 1. Intercept is GM0 and Morph0
> In my understanding the estimates given are the differences from the  
> intercept alone so would give mean values for each category of:
> 1	-1.58481
> 2	-1.5601
> 3	-1.4054
> 4	-3.12549
> 5	-1.80162
> 6	-1.71391
>
> Is this correct? Have I made a mistake in just looking at the  
> difference from the intercept or do 2,3 and 4 need to be taken into  
> account when calculating 5 and 6 i.e. 6 = 1 + 3 + 4?
>

Yes the lower terms need to be taken into account, so 6 is 1+3+4+6.

An easy way of working this out is to create a factor that corresponds  
to the 6 combinations of GM and Morph, fit the model without intercept  
and the values with SE should be calculated automatically.

There is a contrast package but it isn't advertised to work with lmer.

Ken

> I don't want to plough through thinking I've understood this when I  
> may not have! I've tried looking in Pinheiro and Bates etc but they  
> all just say the "estimate is the difference in the means" - I want  
> to make sure I'm interpreting it correctly.
>
> Anyone who can spare this simpleton student a swift reply would  
> really help me out.
>
> ----------------------
> Kate Pressland
> School of Biological Sciences
> University of Bristol
> Woodland Road
> Bristol, BS8 1UG
>
> Kate.Pressland at bristol.ac.uk
>
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