My brain is spinning with all the reading I've been doing on this subject. Can you please explain to me why I couldn't grade within my separate size ranges by grading, for example, 3-6 with one particular grade rule and then taking then changing my grade rule for the next section? Would this not be taking the difference growth rates and such into account? I feel like there's probably a reason that I haven't quite grasped yet. Thanks so much.Grading women's clothing is different from grading children's clothing. Women's clothing is graded with either a 1 1/2" or 2" grade between the sizes. This grade rule refers to the change in circumference measurements between the sizes. There is a lot of criticism for this grade rule because it appears to not have a connection to actual body measurements. That assumption is not strictly true, but it is true this practice is a convenience for the industry. It is too difficult to mass produce clothing and create a custom fit for an individual woman. This grading convention has historically proven to fit the most women in the most efficient way.
Men's clothing has many more sizes. These sizes are given size names that correspond to actual body measurements such as waist, pant leg inseam, or neck and sleeve. The circumference difference is usually 1 inch but it may be 2 inches for larger sizes.
Children's clothing is different. Children grow exponentially from birth. Their growth makes it difficult to group children into size groups, but over 70 years ago a measurement study managed to give us enough measurement data to do just that. That measurement data shows that the change between children's sizes is not even. This is true regardless of how sizes are arranged or named. The circumference measurement difference may range between 1/2" to 1" or more. There are similar irregularities in length grades. Children's clothing brands interpret measurements in their own way, so you will see variation in the marketplace.
Speaking to this particular question. If you group your children's sizes so they are called 0-3M, 3-6M, 6-9M, etc., your grade rule will still vary between the sizes. But there will be no grade change between a 3M and a 6M in the size called 3-6M. When developing a grade rule for this kind of size system, I interpret the sizes like this:
0-3M = 3M
3-6M = 6M
6-9M = 9M
You can call your sizes whatever you choose, but for pattern making and grading you have to assign a meaning to that size. Interpreting sizes with a size label of 0-3M as a single size 3M is the easiest way to develop grade rules and it seems to work.