Data items in the EM_3D_REFINEMENT category record details about the class/particle refinement. In random conical tilt, images were assigned angular positions through rotational alignment and tilt-angles. From each different class, a three-dimensional preliminary model is constructed. To improve the output, those preliminary models from each class that have a high degree of similarity are merged. In theory, these models corresponded to groups of the same molecule just viewed from different orientations. Once all the good random conical tilt models (and their corresponding particle data sets) have been merged, iterative angular refinement is used to improve the model's resolution. Equidistant projections are first generated from the merged model. The entire particle data set (whether the old random conical tilt experimental particles, or new untilted experimental particles, or both) is then cross correlated to each reference projection. A correlation coefficient is generated between each experimental particle and reference projection. For each individual experimental particle, it is matched to the reference projection that gave the highest correlation coefficient. Therefore, it is assumed that this particle matches the Euler angles of the reference projection.