Act on exactly one change before cycling again

Carry one specific modification into the next cycle — not a wholesale overhaul.

Why it works

Changing multiple variables at once destroys what the PDSA loop is designed to produce: knowing which change caused which outcome. A single-variable adjustment keeps cause and effect legible. It also limits the cognitive load of the cycle, making it sustainable across dozens of iterations rather than something done once and abandoned.

How to do it

1. From your Study phase, identify the single change most likely to close the gap.
2. Carry exactly that change into your next Plan — resist bundling in other improvements.
3. After the next Do, study whether that specific change produced the expected effect.

Evidence

Single-variable experimentation is a basic principle of controlled inquiry; its value for isolating cause-and-effect is foundational to scientific and engineering improvement processes, including Deming’s industrial applications. (mechanistic)

In personal contexts, true single-variable control is impossible; the practice approximates it, giving directional rather than conclusive feedback.

Common mistake

Overcorrecting after a poor result by overhauling everything at once, making it impossible to know which change helped and leaving the next cycle no better informed.