[R] test of significance for nlme coefficients

From: Lindsey Root <lroot_at_umiami.ir.miami.edu>
Date: Sat 05 Mar 2005 - 05:49:54 EST


Hello,

I am using the nlme package to fit an exponential decay model to a longitudinal data set:

> ##Model a variable called RASCH16A as an exponential function ## of an
initial value "A" and a decay rate "K"
> fm2 <- nlme(model = RASCH16A ~ A* (exp(-K*DAYS)),
+ ###A has nonzero fixed and random effects, K has nonzero fixed and random effects

+ fixed = A + K ~ 1,
+ data = rasch,
+ random = A + K ~ 1 | SUBJECT,
+ ##Give a start value for the fixed effects, expressed using the
concatenate function
+ start = c(60, 0.007))

However, 20 of the 161 estimates decay rates are NOT decay rates, but are very small positive growth rates (all less than a standard deviation (.03) from zero). I would like to run a significance test to see how many of these positive growth rates are significantly different from zero. However, the nlme package only offers confidence intervals for the model parameters, not the person-specific coefficients. I would welcome any and all suggestions.

Thanks if you can help,
Lindsey Root



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