We discuss the application of the so-called parameter b for the characterisation of hydrogenated amorphous and microcrystalline silicon samples. This parameter which reflects the ratio of the electron to hole mobility-lifetime products results from excess-carrier properties. It was suggested that it is useful because b also monitors the Fermi level position, which is a thermal-equilibrium property. Our study for both doped and undoped amorphous silicon samples confirms the trend that b increases when the Fermi levels moves towards the conduction band. Despite this general trend we do not find a one-to-one relationship between b and the Fermi level. A determination of the Fermi level from the dark conductivity as the corresponding equilibrium property appears to be more reliable.