parameter "k" with witch function sd[k]->minimum

kaiwan

Hi! I have to find the best parameter "k" with witch function sd[k]->minimum. I do that with FindMinimum. "sd" if a function that calculates the standard deviation from my experimental and model data. The model data is obtained from the mathematical model, first order differential equation...which I solve using NDsolve. The problem is that I cant make NDSolve work under FindMinimum.

Something like this:
First, I define function that hace to solve the equation c'[t] == -k c[t], where "k" is a variable. This gives me model data later on. I define it as a function cause I dont know the value of "k" yet.

Then I define another function which calculates the standard deviation:
sd[k_]:= function(model data, experimental data)

Still I have no concrete results.

Then I use FindMiniumm for finding the "k" that gives the smallest Standard Deviation
FindMinimum[sd[k], {k, 0.04}]

And her is the problem, I cant make this combination to work .... help please? Many thanks!