The theoretical investigation of differential, integrated elastic, momentum transfer, viscosity, inelastic, grand total and total ionisation cross sections along with the Sherman functions for the scattering of electrons and positrons by polar NH3 molecule in the energy range 1 eV–1 MeV are reported in the present work. We carry out calculations using the independent atom model (IAM) and IAM with screening correction. The scattering observables are calculated using a complex optical model potential in the framework of the Dirac partial wave analysis. The application of screening correction to the IAM substantially improves the cross sections particularly at lower incident energies. The results obtained from the present study are compared with the available experimental and theoretical works found in the literature. [ABSTRACT FROM AUTHOR]