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*To*: Stefano Trapani <stefano@if.sc.usp.br>*Subject*: Re: [ccp4bb]: 3Fo-2Fc maps*From*: Edward Berry <eaberry@lbl.gov>*Date*: Tue, 28 Nov 2000 11:44:51 -0800*CC*: ccp4 Bulletin Board <ccp4bb@dl.ac.uk>*Organization*: Lawrence Berkeley Laboratory*References*: <Pine.SOL.3.96.1001128161017.11169A-100000@ultra3000.ifqsc.sc.usp.br>*Sender*: owner-ccp4bb@dl.ac.uk

*** For details on how to be removed from this list visit the *** *** CCP4 home page http://www.dl.ac.uk/CCP/CCP4/main.html *** Stefano Trapani wrote: > > So, can I say (very crudely) that a (3mFo-2DFc) map is better than a > (2mFo-DFc) map, in that A very simplistic point comes to mind- this (incrementing the coefficients) is going in the direction of subtracting large numbers to get a small difference and so will increase noise, which at some point must outweigh the advantage of lower bias. Iff we can assume from final R-free values that average total error (random + systematic) of a structure factor amplitude is around 20%, when you double Fo and subtract Fc (~=Fo) the error becomes 40%, when you triple Fo and subtract 2Fc the error becomes 60% so A. you do not want to make a 10Fo-9Fc map B. If the weighting factors do a good job of removing bias already in the (2mFo-DFc) map, then the (3mFo-2DFc) map might be less interpretable due to increased noise. I'm sure others will have more quantitative and theoretically well-grounded advice on this question. Ed

**References**:**[ccp4bb]: 3Fo-2Fc maps***From:*Stefano Trapani <stefano@if.sc.usp.br>

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