Estimation of transfer functions: underbiased or overbiased?

The author considers the estimation of time-invariant transfer functions in a deterministic set-up and under the typical situation where the true system has a transfer function that is more complex than the parametrized model transfer function. It is shown that, depending on whether the numerator or the denominator of the transfer function model is chosen to be monic, the corresponding least squares estimate will be overbiased or underbiased, in a frequency weighted sense