I have a longitudinal data set in long format and I want to run
individual regressions. I do this by using the by() function as
follows:
temp <- by(tolerance.pp, tolerance.pp$id, function(x)
summary(lm(tolerance ~ time, data=x)))
This works fine. Coefficients for the first two subjects are shown
below.
Coefficients:
Estimate Std. Error t value Pr(>|t|)
(Intercept) 1.9020 0.2519 7.549 0.00482 **
time 0.1190 0.1029 1.157 0.33105
Coefficients:
Estimate Std. Error t value Pr(>|t|)
(Intercept) 1.14400 0.13336 8.578 0.00333 **
time 0.17400 0.05444 3.196 0.04948 *
Each subject has five rows of data, measured at periods 0,1,2,3,4. There
are 16 subjects so 80 total rows and the output is 16 regression models.
However, if I put this inside another function, the by() function no
longer generates regressions by subject id. It does generate 16 models;
but each model is identical which is a regression using all 80 rows at
once. The function and coefficients for the first two models are shown
below.
test <- function(X,x,y,id)
{
temp <- by(X, id, function(z) summary(lm(y~x,data=z)))
temp
}
attach(tolerance.pp)
test(tolerance.pp, time, tolerance, id)
>
Coefficients:
Estimate Std. Error t value Pr(>|t|)
(Intercept) 1.35775 0.08952 15.167 < 2e-16 ***
x 0.13081 0.03655 3.579 0.000596 ***
Coefficients:
Estimate Std. Error t value Pr(>|t|)
(Intercept) 1.35775 0.08952 15.167 < 2e-16 ***
x 0.13081 0.03655 3.579 0.000596 ***
Without attaching the data I've even tried
test(tolerance.pp, tolerance.pp$time, tolerance.pp$tolerance,
tolerance.pp$id)
but get the same results. Any thoughts are greatly appreciated.
Russell Ivory
Merrick Bank
South Jordan, UT
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