A named vector of NONMEM model parameter estimates, or in the case of all,
a list of named vectors.
est returns a vector of THETA values.
se returns a vector of THETA standard errors.
rse returns a vector of THETA relative standard errors (se/est*100).
95ci returns a vector of the asymptotic 95% confidence intervals for the elements of THETA (est +/- 1.96*se).
all returns all available THETA information as a list of named vectors.