Hello,
I am a SAS beginner, so please excuse me if my question is silly.
I'm trying to estimate the parameters of a ODE system with four equations and eight parameters. Each of the 4 ODEs look something like this:
dxi/dt = f(param1, param2, param3, param3, input1, input2, input3, input4, x1, x2, x3, x4)
I have a data set that includes the values of the input variables and the dependent variables (x1, x2, etc.) at each discrete time point (t=1, ... 60). Thus, the data is a table that has 8 columns and 60 rows.
If I manually choose the parameter values, I am able to successfully simulate the model using something like this
PROC MODEL
...
SOLVE x1 x2 x3 x4 / time = ...;
RUN;
However, when I try to estimate the parameters, I get an error: "At OLS Iteration 0 there are 1 nonmissing observations available. At least 3 are needed to compute the next iteration."
I also get a warning: "Missing value encountered for initial DERT.x1 at ..." In my data, the input variables are not known at each time but I get the same warning even when I fill the missing values with some arbitrary numbers.
My code is:
PROC MODEL
DATA=... /* the data includes, time, input and actual values of x */
OUT=...
OUTALL;
parameters param1=... param2=... ...;
dependent x1=... x2=... ... ;
DERT.x1 = ...
...
DERT.x4 = ...
fit x1 x2 x3 x4 / time=... dynamic SUR;
RUN;
Can anyone tell me what I'm doing wrong?
Thank you!