Simple is always better. Best to tackle new projects first thing in the
morning instead of late at night.
Thanks for opening my eyes, Joseph.
Steve Rothenberg
Date: Wed, 28 Jan 2009 12:03:55 +0900
From: "Joseph Coveney" <[email protected]>
Subject: st: Re: nested xtmixed variance component model syntax
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Steve Rothenberg wrote (excerpted):
I am trying to model these data:
DepVar = weight
fixed IndVars:
time
geolocation
treatment
I observe the weight of subjects given 1 of 4 different fractions of a
complex contaminant (control [saline], soluble, insoluble and total). This
variable is called treatment. The contaminant was taken from three
different geographical locations (variable: geolocation). The subjects are
observed over two different time periods before weighing, each subject
belonging to one or the other time periods (variable: time).
The design assigns four subjects to the control treatment condition of each
geolocation and time and six subjects to each other
treatment-geolocation-time condition.
As I see it, I have subjects nested within treatment, treatment nested
within geolocation and geolocation nested within time. There are no
repeated measurements on subjects.
[excerpted] I keep running
into non-convergence problems with my various formulations of the random
effects.
[remainder redacted]
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If each animal is observed at only one of the two time periods, then this is
a
factorial ANOVA: all of the factors are crossed, and there's a single error
term, the residual. Each animal is is assigned at random only once to each
combination of factors: treatment group, time group and geographic location
group.
First, balance your design by assigining an equal number of animals to each
cell (combination of predictors), and then use -anova- (or -regress-).
anova weight treatment time treatment*time geolocation ///
treatment*geolocation time*geolocation treatment*time*geolocation
Joseph Coveney
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