A landmark Harvard Business Review article that identifies four implicit rules underlying all TPS activities. Won the Shingo Prize for Research. Steven Spear (MIT) and H. Kent Bowen (Harvard Business School) spent four years studying Toyota's operations and distilled the system into four rules:
1. Rule 1: All work shall be highly specified as to content, sequence, timing, and outcome 2. Rule 2: Every customer-supplier connection must be direct, with an unambiguous yes-or-no way to send requests and receive responses 3. Rule 3: The pathway for every product and service must be simple and direct 4. Rule 4: Any improvement must be made in accordance with the scientific method, under the guidance of a teacher, at the lowest possible level in the organization
The article's insight: TPS's power comes not from the visible tools (kanban, andon, five-whys) but from the rigidity of the rules governing how work is structured. The paradox — rigid specification enabling flexible adaptation — is the "DNA" that most imitators miss. This framing influenced Mike Rother's toyota-kata and the broader understanding that TPS is a thinking system, not a toolkit.