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quadratic factor
2 messages · Lingbo Li, Simon Blomberg
To include a quadratic term, use I(Dor^2) instead of Dor^2. The problem is that R interprets Dor^2 as a formula term ie the main effect as well as the second-order interaction of Dor with itself (which is just Dor). Using the I() construct tells R to treat the enclosed expression as a mathematical expression. Cheers, Simon.
On Thu, 2008-08-14 at 21:30 -0700, Lingbo Li wrote:
Hello R expert, I have fish length and corresponding otolith size at six times for 30 fish. Firstly, I use a linear mixed effect model to correlate the fish length (L) with otolith size in dorsal radius (Dor). Since the residuals are domeshaped, I then tried to add a quadratic factor. However, as the summary shown below, the quadratic factor was missing in the analysis. WHY? Thank you very much. Lingbo
back<-read.table("back.r", header=T)
attach(back)
names(back)
[1] "Fish" "Treat" "Mark" "L" "Dor" "Dis" "Ven" "Width"
library(nlme)
ldor.lme=lme(L~Dor, random=~1|Fish)
ldor11.lme=lme(L~Dor+Dor^2, random=~1|Fish)
summary(ldor11.lme)
Linear mixed-effects model fit by REML
Data: NULL
AIC BIC logLik
1539.336 1552.063 -765.6682
Random effects:
Formula: ~1 | Fish
(Intercept) Residual
StdDev: 31.90245 13.00981
Fixed effects: L ~ Dor + Dor^2
Value Std.Error DF t-value p-value
(Intercept) -30.74635 9.775326 149 -3.14530 0.002
Dor 157.14556 3.157083 149 49.77556 0.000
Correlation:
(Intr)
Dor -0.797
Standardized Within-Group Residuals:
Min Q1 Med Q3 Max
-2.47279716 -0.59191227 -0.09822955 0.49939862 3.12757256
Number of Observations: 180
Number of Groups: 30
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Simon Blomberg, BSc (Hons), PhD, MAppStat. Lecturer and Consultant Statistician Faculty of Biological and Chemical Sciences The University of Queensland St. Lucia Queensland 4072 Australia Room 320 Goddard Building (8) T: +61 7 3365 2506 http://www.uq.edu.au/~uqsblomb email: S.Blomberg1_at_uq.edu.au Policies: 1. I will NOT analyse your data for you. 2. Your deadline is your problem. The combination of some data and an aching desire for an answer does not ensure that a reasonable answer can be extracted from a given body of data. - John Tukey.