Time evolution of the normalized HNCO molar fraction recorded by FTIR spectroscopy (full line) at T = 20 K for a HNCO:NH₃ mixture where NH₃ is largely in excess (1:20 concentration ratio). Fitting the experimental data against the analytical expressions for the HNCO time evolution using a rate equation approach (dashed line), a Monte Carlo approach (dotted-dashed line) and a gamma probability distribution (dotted line) gives the value for the reaction rate at a fixed temperature. The three approaches perfectly fit the experimental HNCO decay curve.