After many months, I am now banging my head against the wall because I can't
find a solution to this seemingly trivial problem. Any help would be
appreciated:
I am trying to apply a nonlinear fitting routine to a 3D MR image on a
voxel-by-voxel basis. I've tested the routine using simulated data and
things went well. As for the real data, the fitting routine works
variably. By variably, I mean the following: with a specific set
of starting parameters the routine would work for the first 10 voxels, for
another set of starting values the routine would work for the first 1000 voxels
and so on. NEVER was I able to get starting values that would allow the
routine to run entirely! The error I would get after fitting the limited
number of voxels is:
"Error in approx(x, fx, n = 2 * n.pts + 1) :
need at least two non-NA values to
interpolate"
I think the error is from the library Bolstad since I'm using the sintegral
function as part of the fitting equation. I've tried numerous nonlinear
functions including: nlm, optim and nlminb, but all stop after performing the
fit on a limited number of voxels. I took the values for which the
fitting routine stops and I applied different starting values to them and it
works! I'm still using the same voxel/parameter values and I am sure they
are not NA values as per the error!
Ok, I think the problem is clear. any solutions? I am new to this
complex world of statistical analysis.
Your help is very very much appreciated.
Thanks,
Ayman
Montreal Neurological Institute
Montreal, Canada
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