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By Marin van Heel, Michael C. Schatz

Posted 24 Nov 2017
bioRxiv DOI: 10.1101/224402

We are currently facing an avalanche of cryoEM (cryogenic Electron Microscopy) publications presenting beautiful structures at resolution levels of ~3 Angstrom: a true resolution revolution [Kuehlbrandt, Science 343(2014)1443,1444]. Impressive as these results may be, a fundamental statistical error has persisted in the literature that affects the numerical resolution values for practically all published structures. The error goes back to a misinterpretation of basic statistics and pervades virtually all popular cryo EM quality metrics. The resolution in cryo EM is typically assessed by the Fourier Shell Correlation FSC [Harauz & van Heel: Optik 73(1986)146,156] using a fixed threshold value of 0.143 (FSC 0.143) [Rosenthal, Henderson, J.Mol.Biol. 333(2003)721,745]. Using a simple model experiment we illustrate why this fixed threshold is flawed and we pinpoint the source of the resolution confusion. When two vectors are uncorrelated the expectation value of their inner-product is zero. That, however, does not imply that each individual inner-product of the vectors is zero (the vectors are not orthogonal). This error was introduced to electron microscopy in [Frank & Al Ali, Nature 256(1975)376,379] and has since proliferated into virtually all quality and resolution related metrics in EM. One criterion not affected by this error is the information-based half bit FSC threshold [van Heel & Schatz: J.Struct.Biol. 151(2005)250-262].

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