To derive the spontaneous polarization of a sample, the first harmonic signal of the electric force microscope (EFM) must be corrected using the permittivity. This can be deduced from a simple EFM-force model and has been verified in experiments. For thin films of high permittivity, any gap of air between the tip and the sample prevents the accurate measurement of the permittivity. Therefore, the contact mode of EFM has to be chosen for measurements of the spontaneous polarization of such films.