Flow stress data from specimens tested in tension or compression, with grain sizes ranging from 90 μm to 1440 μm have been used to construct Hall-Petch plots. Three different methods were used to obtain the yield strength data: 0.2% offset proof stress; finding the minimum of the second derivative of the flow stress with respect to the strain; and the stress at which the strain after unloading is 0.2%, thus subtracting the pseudoelastic component of the strain introduced by twinning. It is shown that the last method leads to more self-consistent results. It is also shown that the usual method of finding the Hall-Petch constants through linear regression may sometimes lead to negative values of the friction stress. It is argued that the friction stress should be introduced as a physical constant, calculated from first principles, in order to obtain more reliable values for both the friction stress, 0-0, and the stress intensity parameter, k.