I'm having a problem with a parameter estimation in a non-linear model. I think the culprit is that ode45 (an ode solver in matlab) is not properly solving my ode. It's the in red highlighted part, which very quickly drops and by this seems to ruin the fit.
Also interesting, this happens in all my data (10 patients) so it doesn't seem to be data dependant. I think this is stopping the optimum-fit algorithms from doing their job.
Anyone familiar with this behaviour?
EDIT: I'm working with 94 data points, and it drops between the first and the second point.
EDIT 2: It's solving the following, if this helps anyone:
$\frac{d}{dt} y_{1}(t) = -p_{1}\cdot y_{1}(t)\; + \;p_{1}\cdot p_{2}\cdot Input(t)$ $\frac{d}{dt} y_2(t) = -\left(p_3+y_{1}(t)\right) \cdot y_2(t)\; +\; p_4 \;+\; R_A(t)$
with $R(t)$ a well behaved function of time.