${\displaystyle SSNR(r)={\frac {\displaystyle \sum _{r_{i}\in R}\left|\sum _{k_{i}}{F_{r_{i},k}}\right|^{2}}{\displaystyle {\frac {K}{K-1}}\sum _{r_{i}\in R}\sum _{k_{i}}{\left|{F_{r_{i},k}-{\bar {F}}_{r_{i}}}\right|^{2}}}}-1}$ 

where ${\displaystyle F_{r_{i},k}}$  is the complex structure factor for image k for a pixel ${\displaystyle r_{i}}$  at radius ${\displaystyle R}$ . It is possible convert the SSNR into an equivalent FRC using the following formula:

${\displaystyle FRC={\frac {SSNR}{SSNR+1}}}$ 

• Resolution (electron density)
• Fourier shell correlation