  We present a robust method, as well as a new metric, for the comparison of
permittivity models in terahertz timedomain spectroscopy (THz-TDS). In this
work, we perform an extensive noise analysis of a THz-TDS system, we remove and
model the unwanted deterministic noises and implement them into our fitting
process. This is done using our open-source software, Fit@TDS, available at :
https://github.com/THzbiophotonics/Fit-TDS. This work is the first step towards
the derivation of uncertainties, and therefore the use of error bars. We hope
that this will lead to performing analytical analysis with THz-TDS, as results
obtained from different setups will be comparable. Finally, we apply this
protocol to the study of a $\alpha$-lactose monohydrate pellet in order to give
more insight on the molecular dynamics behind the absorption peaks. The
comparison with simulation results is made easier thanks to the probabilities
derived from the metric.
