Fourier shell correlation ($\mathrm{FSC}(q)=\frac{{\sum }_{\mathbf{k}\in S_q}{F}_1(\mathbf{k})F_2^{\ast }(\mathbf{k})}{\sqrt{{\sum }_{\mathbf{k}\in S_q}|F_1(\mathbf{k}){|}^2{\sum }_{\mathbf{k}\in S_q}|F_2(\mathbf{k}){|}^2}}$)

Fourier shell correlation compares two three-dimensional maps using their complex Fourier transform coefficients in successive reciprocal-space shells. Independent half-map refinement helps distinguish reproducible signal from overfitting; masking and correlated inputs can bias the result.