On 2011-02-28 14:14, Schatzi wrote:
I am not sure how you simplified the model to:
y = a + b(1 - exp(kl)) - b exp(-kx)
I tried simplifying it but only got to:
y = a + b - b * exp(kl) * exp(-kx)
I agree that the model must not be identifiable. That makes sense,
especially given that removing either a or l makes the model work. Can you
please further explain the math though as I am not understanding it? I do
not see you obtained your equation and when I tried to solve using your
equation I got quite different numbers. Thank you.
You can obviously write your function as
f <- f(x, A, B, K) {A - B * exp(-Kx)}
i.e. in terms of *3* parameters. In that form,
it's apple pie for nls().
fm <- nls(y ~ f(x, A, B, K),
start = list(A = 50, B = 60, K = 1)
coef(fm)
xx <- seq(0, 72, length = 101)
yy <- predict(fm, newdata = list(x = xx))
plot(x, y)
lines(xx, yy, col = "red")
Peter Ehlers
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