The previous article set up a four-arm factorial design — attributes on new products, attributes on established products, both, or neither — and left one decision unresolved: if only one new arm can run next, is it C or D? Answering that well requires being precise about what arm D actually estimates, because the obvious-sounding answer turns out to be wrong...
The Tempting, Wrong Framing
It's natural to reach for D as a way to check whether established products' quality improvement also shows up on the new-product side...