<p>When determining the plasticity response of materials using uniaxial tension tests, the true stress versus true strain behavior can only be directly determined up until the onset of necking. To determine the material response at larger strains, it is necessary to extrapolate the true stress versus true strain behavior past necking and up to fracture. This study investigates the sensitivity of the measurement-in-neck method to the choice of surface strain calculation method when deriving the post-necking plasticity response. Tension tests are performed on additively manufactured 316L stainless steel in as-built and heat-treated conditions; the as-built material exhibits a linear hardening rate, while the heat-treated material exhibits power-law hardening behavior. The local surface strains within the necked region are determined via digital image correlation using four different strain calculation methods: The Lagrange, Euler, and Hencky strain tensors plus the stretch associated with an infinitesimal line segment. Plasticity curves are derived using the axial and the effective strains found via each calculation method, and these curves are used to simulate the tension tests utilizing Ansys LS-DYNA. The results show that this method is highly sensitive to the method used to calculate the surface strain, as each method produces widely different extrapolated plasticity models. Only the plasticity models derived using the Hencky strain are able to successfully capture the experimentally observed necking behavior, and utilizing the Hencky effective strain results in increased accuracy compared to the Hencky axial strain. This demonstrates that the surface strain calculation method is a crucial factor for achieving accurate results when using the measurement-in-neck method.</p>