Hi @PhamBao ,
Because you are only looking at a binary response/flag of good/bad it would not really be appropriate to extrapolate (as it rarely ever is in any case). Similarly these markers are only being considered good/bad when they violate the control limit rules, not because they've actually been measured and placed as 'good' or 'bad' (correct me if I'm wrong). Your application of the contour is not appropriate or even really needed - in the case of your data the change from good to bad isn't gradual, its sudden as the X or Y values cross their control limits.
If the latter is true, then I would suggest a few things:
- Find a quality attribute from this process that defines whether or not the product is good or bad (i.e. product purity, defect rate)
- Measure the associated values of that attribute against your X and Y (although these can both be considered X's)
If you need to then look at trying to use experimentation to fill that gap, look at these DoE resources: https://www.jmp.com/doeintrokit/en/index.html .
Thanks,
Ben
“All models are wrong, but some are useful”