[R-sig-ME] (stupid) results interpretation question
Ken Beath
ken at kjbeath.com.au
Tue Jun 9 13:58:13 CEST 2009
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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