Definition
Unmeasured means no answer was collected for a question and model pair; zero means an answer was collected and the brand was absent from it. Rendering the first as the second turns missing evidence into negative evidence.
Why it matters
It corrupts every downstream number quietly. Failed cells counted as zero drag mention rate down, make a healthy month look like a decline, and can trigger alerts about a drop that never happened.
How to measure it
Keep unmeasured as null all the way to the interface and render it as a dash or as "not measured", never as 0%. Report attempted and completed counts side by side so a low completion rate is visible rather than absorbed.