Generally, the effective order of non-linearity k=( delta lg N)/( delta lg I) observed in multiphoton ionisation is equal to the number of photons k0 absorbed except in the vicinity of a resonance, where k deviates strongly from k0 (N is the ionisation probability, I is the field intensity). By taking into account the space dependence of intensity the authors show that k=k0-1 is expected when (k0-1)h(cross) omega is a little smaller than the ionisation energy of the initial state.