Sorry for the delay, but I were very busy.
The DCR is a rating system that help to cost effective design. I guess it deals with the special caracteristic (appendix C), in fact I would like to confirm this. Though all dimension are to be within the specified tolerance, the practice teach us that some needs more attention. The level gives a range of dimensions from the essentiel one to the minor, and further the way of taking into account control or statistical tool. May be statistical technique is not the right topic because it deals with more general view of the design and its specifications ?