Measurements of quenched-in resistivity ( A p O b s )~ as a function of temperature canyield values of the effective enthalpy for vacancy defect formation H:m. The samples are usually made thin to achieve high quenching rates and pure to avoid vacancy-impurity clustering during the quench. The higher the sample purity and the thinner the sample. the more the sample resistance is affected by scattering of electrons from the sample surface. Inadequate correction for this size effect can result in errors in the values of ApQ and thus H:r. The magnitude of the errors using Fuchs-Sondheimer and Dingle theories is discussed. A possible method of obtaining reliable size-effect corrections is suggested by an application of Soffer theory.