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29645
2006 Feb 28
1
Collinearity in nls problem
Dear R-Help list,
I have a nonlinear least squares problem, which involves a changepoint;
at the beginning, the outcome y is constant, and after a delay, t0, y
follows a biexponential decay. I log-transform the data, to stabilize
the error variance. At time t < t0, my model is
log(y_i)=log(exp(a0)+exp(b0))
at time t >= t0, the model is
log(y_i)=log(exp(a0-a1*(t_i - t0))+exp(b0=b1*(t_i -