[R] Within factor & random factor

細田弘吉 khosoda at med.kobe-u.ac.jp
Wed Aug 26 14:28:48 CEST 2009

I am quite new to R and trying to analyze the following data.  I have 28
controls and 25 patients.  I measured X values of 4 different locations
(A,B,C,D) in the brain image of each subject.  And X ranges from 0 to 1.
I think "control or patient" is a between subject factor and location is
a within subject factor.  So,

controls: 28
patients: 25 (unbalanced data set)
respone measure: X values (ranging 0 to 1)
fixed factor: control vs. patient (between subject factor)
random factor: location (level: A,B,C,D ;no order) (within subject factor)
random factor: subjectID 1-53

My data looks like this;

CorP		X	location	subjectID
control		0.708	A		1
control		0.648	A		2
patient		0.638	C		3
control		0.547	D		4
patient		0.632	B		5
control		0.723	C		6

I want to know
(a) if there is a significant difference between controls and patients
in X values.
(b) where (A,B,C,D?) the difference is between controls and patients in
X values.  (There may be an interaction)

I constructed linear mixed model with lme as followings;

(1) model1 <- lme(X ~ CorP*location, random= ~ 1| subjectID, mydata)

(2) model2 <- lme(X ~ CorP*location, random= ~ location| subjectID, mydata)

I am not familiar with lme syntax.  I'm just wondering which formula
[(1) or (2)] is appropriate for my model to know answers of (a) and (b)
questions.  Or may be both of the formulas are wrong.

I would appreciate it very much if somebody could help me.


Kohkichi Hosoda

More information about the R-help mailing list