I finally made my example measurement/grading charts. It took a little dancing as I had to create them in a spreadsheet, export them as a PDF and then convert them over to a jpg. Before I get too far, it may be helpful to review my previous blogs on grading: Creating a grading standard and Creating a grading standard part 2. You should have your own measurement charts handy. BTW, the measurements in my charts are real. I found them from the Sears website. I do not necessarily endorse Sears measurements as the industry standard - they were just handy (To be fair, their measurements are pretty good). Your own measurement charts should be far more complete and detailed than these for pattern development and actual grading anyway. This is the first step I take in developing actual grade rules.
Right off, I hope you notice a few important things. First, I have clearly marked my sample sizes. In this example they are sizes 10 and 10+. Each chart is labeled clearly. You would think these things should be obvious, but you would be surprised at what I have seen. Hanging off of the left side are numbers. These numbers are points of measure and should correspond to a How to Measure diagram (a future blog) and are not relevant for the immediate discussion. Also notice that my size ranges differ. The regular sizes run 7-16 and the plus sizes run 8+ to 18+. This is a fairly typical difference between the two groups. Also notice the difference in the measurements. There should be some obvious differences between a Misses and a Plus sizes chart too.
Usually, I have my grading chart separate from my measurement chart. I combined them here so you could more easily see how I am developing my grading rules. In the column for my sample sizes, I have placed a zero. A sample size is also called a base size in grading. It is your starting point and each subsequent size will grow or shrink proportionally off your base size. Next I subtract the difference between the base size and the next largest size. In the regular size chart, you will notice there is a 1.5" difference in the chest measurement from a size 10 to a size 12. This difference is called a grade step. Next I subtract the difference between a size 14 and a size 12. I don't subtract Size 14 from a size 10 because that is not the next grade step. Continue to subtract the next larger size with the previous size.
To calculate the grade for your smaller sizes, subtract your sample size from the next smaller size. Be sure to add the negative sign, which indicates the grade is getting smaller. Repeat by substracting each size with the next smaller size. Create a grade for each measurement on your chart.
You will notice that the measurements and grades all have beautiful numbers. The measurements increase or decrease proportionally causing the grades between sizes to be relatively the same. This is where the art of grading and measurements come into play. I can guarantee that actual body measurements are not this pretty or consistent. These numbers have been averaged and rounded and are based on a large body measurement sampling. The numbers have been intentionally made easy to work with. Your measurements should be easy to work with too and you can adjust them as necessary.
What about accuracy? Rounding does introduce inaccuracies in your measurement charts, but only a little. If you look at growth charts, you will find that certain measurements will fit 50%, 80%, 95% or 97% of girls. If you adjust your numbers up or down, you will want to make sure those numbers fall into the 95% percentile. Adult measurements and sizing are similarly developed. Because these measurements are based off of measurement studies, it means a real girl will pick the size that most closely matches her measurements. Your measurement numbers just need to fall within the highest percentage category. You can round to the nearest 0.5" or 0.125", or whatever. Adjust your measurements so that you get a relatively consistent and even grade across the sizes. In my regular sizes I have a consistent 1.5" chest grade and a 2" grade for my plus sizes. Sure, you could throw in what ever grade steps you choose as long as you have justification for it.
Finally, I hope you can see the difference between the regular and plus sizes. Not only are plus size measurements larger, they are proportionally larger (the grade step is larger). This is why you absolutely cannot grade a plus size pattern from a regular sized pattern piece. Keep both categories, pattern pieces, measurements and grades separate from each other. I promise it will save your sanity.
I know this is a lot of explanation for this first step. If anyone has a question about this, just leave them in comments and I will try to answer them. In my next article in this series, I will explain how to create grade rules based off your first grading chart.