[R] Measuring correlations in repeated measures data

[Ista Zahn]

Hi Brant,
My version of Orthodont doesn't seem to have as many levesl of age as
yours but the general idea is

> library(MEMSS)
> data(Orthodont)
> library(reshape) # could use reshape function in stats package instead of cast
> c.orth <- as.data.frame(cast(Orthodont, Subject + Sex ~ age, value="distance"))
> names(c.orth)[3:6] <- paste("age", names(c.orth)[3:6], sep="_")
> cor(c.orth[3:6])
           age_8    age_10    age_12    age_14
age_8  1.0000000 0.6255833 0.7108079 0.5998338
age_10 0.6255833 1.0000000 0.6348775 0.7593268
age_12 0.7108079 0.6348775 1.0000000 0.7949980
age_14 0.5998338 0.7593268 0.7949980 1.0000000


Best,
Ista
On Mon, Feb 28, 2011 at 12:07 AM, Brant Inman <brant.inman at mac.com> wrote:
> R-helpers:
>
> I would like to measure the correlation coefficient between the repeated
> measures of a single variable that is measured over time and is unbalanced.
>  As an example, consider the Orthodont dataset from package nlme, where the
> model is:
>
> fit <- lmer(distance ~ age + (1 | Subject), data=Orthodont)
>
> I would like to measure the correlation b/t the variable "distance" at
> different ages such that I would have a matrix of correlation coefficients
> like the following:
>
>      age08 age09 age10 age11 age12 age13 age14
> age08    1
> age09          1
> age10                1
> age11                      1
> age12                            1
> age13                                  1
> age14                                         1
>
> The idea would be to demonstrate that the correlations b/t repeated measures
> of the variable "distance" decrease as the time b/t measures increases.  For
> example, one might expect the correlation coefficient b/t age08 and age09
> to be higher than that between age08 and age14.
> Is there a function that can calculate such correlation coefficients of a
> repeatedly measured variable "y" ("distance" in the Orthodont dataset)
> across some category "x" ("age" in the Orthodont dataset)?
>
> Thanks,
>
> Brant
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-- 
Ista Zahn
Graduate student
University of Rochester
Department of Clinical and Social Psychology
http://yourpsyche.org