[R] nls question

[Mark M. Span]

Just to add a note to the question asked by Bill Simpson..

I am confronted with a similar problem, but the model I try to fit on my
data is nonlinear.
I have a four-parameter, biexponential model describing the response times
of a group of subjects performing a set of task. What I want to evaluate is
whether the parameter values differ between (groups of) tasks.

myfunc    <-formula(x ~ a*exp(-b*age) + (c*exp(d*age)) )

both x (representing response time) and age are variables in the current
scope. Part of the data frame is presented below.  At the moment I fit the
model for each task seperately. But now I cannot test the parameters for
equality. A solution like in Bills case would be great.

         task       type     cor         x        var median     stdev count
pp nper nsw age
266    DS Repetition Correct  581.2427 108609.931  499.0 329.56021   103 007
22  17  19
267    TS Repetition Correct  508.1159  31364.839  461.0 177.10121    69 007
0   0  19
268   SRT Repetition Correct  215.8182  14352.028  188.0 119.79995    33 007
0   0  19
269 CRT-1 Repetition Correct  379.6250   3335.601  382.0  57.75466    48 007
0   0  19
270 CRT-2 Repetition Correct  488.4898  11872.963  461.0 108.96313    49 007
0   0  19
271 CRT-3 Repetition Correct  411.3600   7523.256  393.0  86.73670    50 007
0   0  19
272 D/CRT Repetition Correct  328.0313   1587.709  327.0  39.84606    32 007
0   0  19
273    DS  Induction Correct  616.3548  74128.037  540.0 272.26464    31 007
22  17  19
274    TS  Induction Correct  484.7867  54941.630  438.0 234.39631    75 007
0   0  19
282    DS Repetition Correct  681.6842 225197.191  519.0 474.54946   114 058
14  19  20
283    TS Repetition Correct  646.3538 106069.951  539.0 325.68382    65 058
0   0  20
284   SRT Repetition Correct  217.0833   1823.336  213.0  42.70054    36 058
0   0  20


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