Naydenova, K. & Russo, C.J. Nat. Commun. 8, 629 (2017).

To obtain the highest possible resolution using single-particle cryo-electron microscopy (cryo-EM), protein particles on plunge-frozen sample grids should ideally adopt random orientations. However, many proteins adopt preferred orientations, which limits the achievable resolution. Naydenova and Russo report a statistical metric that they call the 'efficiency', Eod, for characterizing particle orientation distribution. This metric can be calculated using a relatively small number of particles; its value is interpreted to indicate whether data collection can proceed to reach a desired resolution, or whether one should first try alternative sample grids or data collection strategies, such as optimizing the sample tilt angle, to achieve higher resolution. The authors demonstrate the use of Eod for four previously published data sets with a broad range of particle orientation distributions. An open-source software tool is available.