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Item _em_map.num_class_averages

Description

    The number of class averages (one class average contains 
     images that are the same) resulting from the multivariate statistical
     analysis of the individual images of particles are 2d projections
     of a 3d structure in different projection directions. Given a 
     sufficiently large number of good 2d projections the 3d structure
     can be reconstructed knowing the orientational relationship 
     between all the projection class averages. For an entirely asymmetric
     particle at least three different projections are required to solve
     the orientation problem.

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