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Dear all, I am a student in crystallography. So not quite familiar with some even basic concepts. In shelx .hkl file or ccp4 .mtz file there is a column SIGF which is related to standard deviation of the structure factor. I read through many text book for crystallography, there are many formulas about this topic. Sometimes it is a square of sigma, sometimes it is not. My question is: 1. What is the exact mathematical formula for SIGF or SIGFP in ccp4 or shelx format file? 2. If it can be calculated from F, why it is necessary to include it in ccp4 or shelx reflection file (they have F already) ? 3. Is this value really important in structure determination? Why and how? As I understood, during data collection each reflection measured several times, so there is a deviation from the average F. That is the meaning of SIGF. But how to use this value in structure determination? Is there some kind of correction or refinement on F according to SIGF? And also when multiplicity during data collection is low, the SIGF would not be so interested. So is that means the SIGF would not be so important in some measurements? Any kind reply from you guys would be greatly appreciated. Many thanks! Best regards, G