where "S = dP/dh" is the apparent tip-sample contact stiffness at the onset of unload, formula_103 is the displacement rate just before the unload, formula_104 is the unloading rate, and formula_105 is the true (i.e. viscosity-corrected) tip-sample contact stiffness which is related to the reduced modulus formula_7 and the tip-sample contact size formula_68 by the Sneddon relation . The contact size a can be estimated from a pre-calibrated shape function formula_108=formula_109 of the tip, where the contact depth formula_10 is obtainable using the Oliver—Pharr relation with the apparent contact stiffness formula_6 replaced by the true stiffness formula_105: