(stupid) results interpretation question
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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