Issue with 95% CI using MASS{confint}
On 27/05/2015 4:08 PM, Jennifer Sweatman wrote:
I am trying to run a nonlinear model looking at seasonal abundance of
plants with the following limitations:
fit.nls.s<-nls(S_a~beta+m*time+alpha*sin(w*time+phi),
data= xy,
start=list(beta=2.1, m=0, alpha=2, phi = 1, w = 1),
upper=list(beta="inf", m="inf", alpha="inf", phi = 2*pi, w =
"inf"),
lower=list(beta="-inf", m="-inf", alpha=0, phi = 0, w =
0.0000001),
algorithm="port")
I want to know the 95% confidence intervals for all of my model
parameters. I have been using confint from the MASS package but I?m
getting some NA values. See output below:
confint(fit.nls.s)
2.5% 97.5%
beta 1.68170858 2.5635734
m 0.02810172 0.0710163
alpha NA 0.6755750
phi NA NA
w NA 1.0275199
The parameters I most need 95% CIs for are alpha and w (when alpha is
significant), but I consistently get NA for the lower limit of both. I
have changed my Upper/Lower limits for alpha, phi, and w to +/- infinity,
but my CIs still don?t include complete intervals. Any suggestions as to
what is going on here?
Probably the profiling failed. You could look at plot(profile(fit.nls.s)) to see an indication of what's going on. If your tau values don't exceed 2 on one side of the estimate, you can't get a confidence limit on that side. Your data may not rule out any phi values, or may not rule out a lower limit on alpha or w. Duncan Murdoch