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Oops, I meant if the variance of the X-ray background is exactly zero (and not if the X-ray background is zero). Think about that in the context of Poisson counting statistics. :) ________________________________ From: CCP4 bulletin board [[log in to unmask]] on behalf of Jim Pflugrath [[log in to unmask]] Sent: Thursday, August 23, 2012 12:44 PM To: [log in to unmask] Subject: Re: [ccp4bb] Calculating I/sig when sig = 0 Singly-measured reflections should have a sigma from Poisson counting statistics, so that should not be a problem. A problem might occur if the X-ray background is exactly zero and the observed (sic) intensity is exactly zero. ________________________________ From: CCP4 bulletin board [[log in to unmask]] on behalf of Jacob Keller [[log in to unmask]] Sent: Thursday, August 23, 2012 12:36 PM To: [log in to unmask] Subject: [ccp4bb] Calculating I/sig when sig = 0 Dear Crystallographers, what approach is taken to calculate I/sig when sig = 0? (This could happen for singly-measured reflections or perhaps some other scenario, such as rejection of other measurements leaving only one measurement.) I could imagine alternatives, but what is actually done? JPK -- ******************************************* Jacob Pearson Keller Northwestern University Medical Scientist Training Program email: [log in to unmask]<mailto:[log in to unmask]> *******************************************