Some errors can be classified in many ways, and by grouping simple errors it would be possible to give a more precise explaination on what has been played. For example, rhythm errors on fast and regular notes are now handled as many consecutive ``notes too short'' and ``notes too long''. But it makes more sense to classify them as only one error, which will be defined as a ``lack of regularity''.
In that way, it would be a good idea too make a second classification after the first one, as a kind of supervising mechanism which would be looking for a logical link between played errors.
It would be great, in the future, to think about a 3-phased diagnostic system: detection - classification - grouping.