Average figure of merit of phases of reflections included in the refinement. This value is derived from the likelihood function. FOM = I₁(X)/I₀(X) I₀, I₁ = zero- and first-order modified Bessel functions of the first kind X = sigma_A |E_o| |E_c|/SIGMA E_o, E_c = normalized observed and calculated structure factors sigma_A = <cos 2 pi s delta_x> SQRT(Sigma_P/Sigma_N) estimated using maximum likelihood Sigma_P = sum_{{atoms in model}} f² Sigma_N = sum_{{atoms in crystal}} f² f = form factor of atoms delta_x = expected error SIGMA = (sigma_{E;exp})² + epsilon [1-(sigma_A)²] sigma_{E;exp} = uncertainties of normalized observed structure factors epsilon = multiplicity of the diffracting plane Ref: Murshudov, G. N., Vagin, A. A. & Dodson, E. J. (1997). Acta Cryst. D53, 240-255.