The paper studies the error in methods of calculating the transmission function of atmospheric gases using a statistical model of the absorption band. A direct implementation of this approach in a very simple program and the well known software package ModTran are considered. The results of comparison of calculations using these methods with direct (linewise) calculations, used as a reference, are given. The absorption bands of four gaseous components are studied: H₂O, CO₂, O₂ and CH₄, in the near-IR area of the spectrum (0.6-2.5 μm). It is concluded that calculations using the ModTran software are highly accurate, so that it can be used to solve a number of inverse problems of atmospheric optics.