I am author of the first book (by a non-Japanese) featuring lean manufacturing, at first known as just-in-time manufacturing: Japanese Manufacturing Techniques: Nine Hidden Lessons in Simplicity (1982, Free Press). Later, the business press, plus consultants and authors, were inclined to rename the methodology as just-in-time inventory. That label relates closely to the Reaction Gauge question as to "insufficient inventory" as the/a cause of "empty shelves." But just-in-time "inventory" is not the essence of lean/JIT: "customer pull" is much closer to the mark. But manufacturers who aspire/claim to be bastions of customer pull usually are far from it. What's missing, mostly, is "flexible response." Those ostensibly lean producers take high pride in their takt-time schedules, while generally obeying the conventional siren song of high resource (people and equipment) utilization--in the belief that lean must reduce (in-process) costs (and not just inventory costs). What's sorely missing is flexibility to easily and quickly adapt to always unpredictable demand. To do that requires simple, low-cost facilities in multiples (the cellular/focused plant-in-a-plant concept)--no monuments--such that there is plentiful redundant capacity in the form of low-utilization, even some mostly idle, cells. I've written many articles about this, one label of which is "concurrent production." For further reference please see my (rather crude, rarely updated) website, www.wcm-wcp.com.