The probabilistic formula derived via the rigorous method of the joint probability distribution function to estimate protein phases in the single-wavelength anomalous diffraction (SAD) case [Giacovazzo & Siliqi (2001). Acta Cryst. A57, 40-46] has been revised. A simple but equally effective formula is provided, allowing an easy interpretation of the role of the structural parameters accessible via a diffraction experiment. In particular, the formula is able to simultaneously combine the contribution arising from the anomalous differences with a Sim-like contribution, and also to take the errors into account.