Answer:
as soon as there is a design to improve
Explanation:
As a design engineer, I started on the "design improvement" step as soon as I had an initial conceptual design.
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Then, I started that step again when my boss told me, "make it better."
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The more interesting question is, "when do you stop the design improvement step?" (Judging by the constant barrage of software updates, that answer is, "never.")
