This computes the first derivative of the D-optimality criterion with respect to the model parameters for a specified model.
smalld1(T, x, xl, inv, order)
A numeric vector. Model parameter values
A numeric. ith dose level
A numeric. last dose level
A numeric matrix. Inverse infromation matrix
numeric. The number of model paraemters: 3, 4, 5 for the 3, 4, 5PL models respectively