An analysis Simchi-Levi did recently with executives at Ford Motor Co. confounded that notion. It showed that some of the lowest-cost items, from minor suppliers, can cause the biggest and costliest disruptions.

Examples related to making cars include such things as valves (costing a few dollars) or o-rings (costing a few cents). In many cases, he said, companies have alternative suppliers and ample inventories for big-ticket parts but no such backstops for bits and pieces whose absence nevertheless could halt production.

"Risk is hidden in unexpected places," Simchi-Levi says.

Manufacturers around the world got a crash course in supply-disruption risks in the wake of Japan's earthquake and tsunami and the Thai floods in 2011. Those disasters cut off vital parts used globally for such things as cars, construction equipment and electronic devices.

It's impossible for companies to make regular visits to assess risks at every single supplier, especially those they don't deal with directly. So Simchi-Levi and colleagues have developed computer models to search for weak points. Those models help find situations in which a company is relying on a single supplier and where inventories are small or capacity insufficient.

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